Common knowledge is what "everybody knows", usually with reference to the community in which the term is used.
The assertion that something is "common knowledge" is sometimes associated with the fallacy argumentum ad populum (Latin: "appeal to the people"). The fallacy essentially warns against assuming that just because everyone believes something is true does not make it so. Misinformation is easily introduced into rumors by intermediate messengers.
In many cases "common knowledge" is also widely established as being true. So then the assertion that something is "common knowledge", is merely another way of saying that someone is ignoring an obvious fact, or should have known about it had they been paying attention.
Many techniques have had to have been developed in response to the question of distinguishing truth from fact in matters that have become "common knowledge". The scientific method is usually applied in the cases involving appeal to and familiarity with the phenomena associated with astronomy, mathematics, physics, and as such to the general laws of nature. In legal settings, rules of evidence generally exclude hearsay (which may draw on "facts" someone believes to be "common knowledge"). To avoid undue influence from publicity on the neutrality of a jury, judges occasionally must order a change of venue. Journalists and other investigators of truth may seek a source closer to the facts, eliminating unreliable intermediaries.
That set of truths amenable, discoverable and convenient to use which have come to be known as such by an informed group of parties interested in their own general circumstances, the circumstances each of their own particular endeavours and studies, and the general circumstances pertaining to such studies and endeavours is called amongst them the body of common knowledge, and such informed groups being usually associated strongly by kinship, locality nationhood or like primary affiliation and as are also numerous and inventive in such number then : "common knowledge" rightly is known, to be the body of common knowledge held to be as such within that larger group. . "Conventional wisdom" is a similar term, coined by economist John Kenneth Galbraith, referring to ostensibly pervasive knowledge or analysis.