MSM is an acronym for "Main Stream Media". In common usage it refers to both print and broadcast sources of news, current affairs commentary and entertainment with the term "Mainstream" applied to distinguish it from so-called "Alternative" media.
Definition and Demarcation
Both the definitions and demarcation, as between "Mainstream and "Alternative", are necessarily somewhat arbitrary. They apply according to where, on a spectrum between Establishment sycophancy and outright Establishment enmity, the editorial policy (stated or otherwise) of a particular publishing organisation sits. They are UNCONNECTED with where on the traditional Left-Right political spectrum it sits, the entire spectrum having long-since been co-opted and absorbed by the Establishment. Historically the Left has been more problematical for Establishment interests but, in its overtly anti-establishment forms, it too has been more or less successfully neutered following the demise of the old Soviet Union.
Content Policing and the MSM self-image
Most MSM and MSM professionals, undoubtedly regard themselves, at the very least, as Establishment sceptic and, on matters unconnected with 'Deep State' issues, they are mostly both honest and correct to do so. However the boundaries of allowable debate and discourse, though largely unstated, MUST be respected if career progression within the MSM structure is to remain open. The archive section of the 'Media Lens' web site provides numerous trenchant illustrations of how this content policing operates. 
There are many taboo subjects and knee-jerk buzz-words which the ambitious journalist/commentator/celebrity knows he/she must navigate with extreme caution. Among the latter are: "Holocaust", "Conspiracy", "Anti-Semitic", "Zionist", "Nazi", "Terrorist"; among the former, dissent about the merits of: Globalisation, Economic Growth (on a finite planet), Free-Trade (so-called), and Western definitions of "Freedom" and "Democracy" - all of which are treated as articles of faith to be questioned only on pain of excommunication and severely stunted career prospects. Similarly risky/taboo behaviour is to question: The real motives of US/UK/NATO military entanglements; the alleged (assumed) benign intent of Western geo-policy, the essentially defensive nature of the wars in Iraq and Afghanistan; the reality and extent of 'the terrorist threat' - and a good few more besides.
In broad terms, "mass circulation/audience" and "mainstream" are synonymous. Trans-national corporate interests dominate MSM organisations ownership (which are themselves Zionist dominated). Where such corporate interests do not own outright - and with the notable exception of the BBC (See below) - they provide the dominant income stream through advertising.
On the face of it the BBC, as a tax-payer funded organisation (or 'public-service broadcaster' as current Newspeak  has it), ought to be less susceptible to the pressures that steer and moderate content in the vast bulk of the MSM as outlined above. But appearances can be deceptive.
There is no doubt that, in spite of left-leaning tendencies, in the dog-fight of domestic party political trivia, the BBC is indeed relatively unbiased, balanced and impartial. Unfortunately, this makes it all the more sensitive to charges that, at a more fundamental level on matters central to the modern Western 'progressive' creeds outlined above, all may not be as it seems and it is therefore inclined simply to dismiss charges of institutional bias (intentional or otherwise), out of hand. There are many cases of such lofty dismissal, amply documented, on the Media Lens Web site mentioned above .
Where issues impinge upon 'Deep-State', Permanent Government interests, the BBC has always been the voice of the British Establishment. Its charter is the work of men wedded to the Mackinder-Rhodes-Milner vision of the British Empire as a missionary force for 'progress' and the spread of civilisation in the world; and it is financed on the whim of the British government whose hidden, permanent and secret elements dominate. How could it be anything else?
Subservience to Deep State interests
The 1926 General Strike
John Reith, then general manager and later Director General of the BBC, confided to his diary: "The Cabinet decision is really a negative one. They want to be able to say that they did not commandeer us, but they know they can trust us not to be really impartial."  Reith is also quoted as saying: “since the BBC was a national institution, and since the government in this crisis was acting for the people...the BBC was for the government in the crisis too.” 
The BBC began in 1922, just before the corporate press began in America. Its founder was Lord John Reith, who believed that impartiality and objectivity were the essence of professionalism. In the same year the British establishment was under siege. The unions had called a general strike and the Tories were terrified that a revolution was on the way. The new BBC came to their rescue. In high secrecy, Lord Reith wrote anti-union speeches for the Tory Prime Minister Stanley Baldwin and broadcast them to the nation, while refusing to allow the labor leaders to put their side until the strike was over. So, a pattern was set. Impartiality was a principle certainly: a principle to be suspended whenever the establishment was under threat. And that principle has been upheld ever since.
A latter day example
A demeaning latter-day example of BBC subservience was provided by the treatment accorded Andrew Gilligan, one of its reporters, over his broadcast claims about the 'sexing up' of the 'Iraq Dossier' used to justify the UK's participation in the invasion of Iraq and which led to the death of UN Weapons Inspector Dr David Kelly. In spite of Gilligan's claims being largely vindicated by subsequent revelations, he was forced to resign from the BBC, as was its Chairman Gavyn Davies and its Director General Greg Dyke. 
Control by Deep State interests
Further evidence of the covert control exercised over the BBC throughout the post WWII period to about the mid 1980's is provided by widely reported and well substantiated revelations of the systematic MI5 vetting of senior (and not-so-senior) BBC appointments and projects. See the WikiSpooks Document "MI5 and the Christmas Tree Files" for full details. 
BBC rejects humanitarian appeal
In the 2008/2009 attack on Gaza by Israel known as Operation Cast Lead at least 1,417 people (mostly women and children) were killed and more than 3,000 homes and hundreds of other properties, including factories, workshops, animal farms and orchards, as well as government buildings, police stations and prisons, were destroyed and more than 20,000 were damaged.
"The level of human suffering and destruction I saw today is, from any angle, heartbreaking. It is shocking that civilians suffered so disproportionately in this military operation". UN Under-Secretary General for Humanitarian Affairs and Emergency Relief Coordinator John Holmes, 22 January 2009.
The Disasters Emergency Committee launched a public appeal to support its humanitarian relief efforts in Gaza.  The BBC, alone among the UK MSM refused to broadcast it on grounds of "Upholding the BBC's hard won reputation for impartiality" and stood firm in its decision despite widespread criticism.  The Archbishop of York, John Sentamu, accused the broadcaster of taking sides, "This is not a row about impartiality but rather about humanity". Communities secretary Hazel Blears said: "I sincerely hope the BBC will urgently review its decision."
Whilst inquirers will undoubtedly be informed that these were all unfortunate aberrations from which lessons have been learned, there is every reason to suppose that, just like the Ronnie Corbett character in that John Cleese-Ronnie Barker-Ronnie Corbett video sketch, when it comes to deep political issues of the British Establishment, the BBC continues to "know its place". 
New York Times
Despite the generally high quality (and excellent reputation) of this source, there are severe problems in any reports it publishes on the Israel/Palestine topic.
The New York Times "owns" a property in the prestigious Qatamon neighborhood of Western Jerusaleam. It was once the home of Hasan Karmi, a distinguished BBC Arabic Service broadcaster and scholar. Karmi was forced to flee with his family in 1948 as Zionist militias ethnically cleansed Arab neighborhoods. An estimated 10,000 Palestinian homes in West Jerusalem were stolen that year. Hasan Karmi’s daughter, Ghada, a physician and well-known author in the United Kingdom, discovered that The New York Times was in - or rather on top of - her childhood home in 2005, when she was working temporarily in Ramallah.The NYT correspondent, Ethan Bronner, actually lives in this stolen house. (How he can be neutral in his reporting of the subject is extremely difficult to understand, since his son serves in the Isdraeli forces). However, he is fully aware of the situation and is quoted as being uncomfortable about it:
"One of the things that is most worrying not just the Left but a lot of people in Israel about this decision is if the courts in Israel are going to start recognizing property ownership from before the State [of Israel was founded]," Bronner said according to a transcript made by independent reporter Philip Weiss who maintains the blog Mondoweiss.net. Bronner added, "I think the Palestinians are going to have a fairly big case. I for example live in West Jerusalem. My entire neighborhood was Palestinian before 1948."
Wikipedia and its use of sources
The Wikipedia policy of "Reliable Sources" is a two-edged sword, wherein articles will take the same line as originally published in the Western MSM. What happens once the media circus has moved on is naturally covered much less thoroughly. Major errors subsequently uncovered can be included, but will often be ignored because they were likely found by little-known local reporters and, in international news, published in non-English sources. Biased editors can very easily block inclusion of the wider or corrected picture.
In many cases involving stories based in the English-speaking West (and not involving goverment mal-feasance), the Wikipedia method is perfectly adequate, but it is a major failing where international affairs or serious criticism of government are involved.
The MSM also suffers from bias introduced by media owners. Human rights stories from China will almost inevitably be mis-reported, because that particular tightly controlled market is so important to most of the media. Rupert Murdoch is noted for having invested heavily there (though not always successfully).
Lastly there is something that will greatly suprise most people, Wikipedia also has an owner, and he has no hesitation in heavy-handed interfering. Some of the alarming consequences of this are described at another Wikispooks article, Wikipedia's Hasbara.
Proposed to heavily modify this original section, or blend into the above:
With a casual readership that dwarfs the largest MSM publications, co-option of Wikipedia to the serried ranks of the MSM was all but inevitable and so it has turned out. 
Wikipedia has a policy of "Reliable Sources", known as WP:RS. Unfortunately, its concept of questionable sources is defined in ways very much biased towards the understanding of western readers - on top of this is the problem of highly uneven enforcement. Policies such as this weigh against even eye-witness evidence unless it has been published by western sources.
- Media Lens archive
- Wikipedia page on George Orwell's "Newspeak"
- C. Stuart (ed.) The Reith Diaries (1975)
- cited in Michael Gurevitch, Culture, Society, and the Media (Routledge, 1982) p.302
- The Invisible Government, John Pilger, Information Clearing House, Speech delivered at the Chicago Socialism 2007 Conference on Saturday June 16 2007
- Andrew Gilligan Wikipedia page
- "MI5 and the Christmas Tree Files"
- Palestinian Centre for Human Rights (19 March 2009). "Confirmed figures reveal the true extent of the destruction inflicted upon the Gaza Strip; Israel’s offensive resulted in 1,417 dead, including 926 civilians, 255 police officers, and 236 fighters.".
- Amnesty Report on "Operation Cast Lead" more than 3,000 homes and hundreds of other properties, including factories, workshops, animal farms and orchards, as well as government buildings, police stations and prisons, were destroyed and more than 20,000 were damaged. July 2009.
- [http://www2.ohchr.org/english/bodies/hrcouncil/docs/12session/A-HRC-12-48.pdf Goldstone Report on "Operation Cast Lead".
- Disaster Emergency Committee Appeal for Gaza
- BBC refusal to broadcast DEC appeal - The Guardian 24 Jan 2009
- Media Lens of the BBC refusal to broadcast the DEC appeal for Gaza
- BBC crisis over refusal to broadcast Gaza appeal 24 January 2009.
- John Cleese & The Two Ronnies video sketch 'I know my place'
- "My entire neighborhood was Palestinian before 1948" Ethan Bronner, Jerusalem correspondent of the New York Times quoted by Philip Weiss, cited by Ali Abunimah of the Electronic Intifada, 2 March 2010.
- Wikipedia as an MSM organisation - The Problem with Wikipedia