Although the context can dictate the term applicable to art and culture, as jno describes, the term, usually applied by insider elitists at academic ivory towers addresses a transformation in defining truth,(in all areas of life). Their definition of truth is "nothing, according to the postmodernist, can be determined to be "true", except postmodernism. The attempt, again, emanates primarily from liberal univesities (at least here in the U.S.)
From the early 1900's until about 1980 or so, it was accepted that truth could be defined and was universally recognized by certain traits. Beginning with the French (of course) deconstructionists who maintained there is no ultimate truth, other than that which the individual determines for him/herself the creed Your truth is as valid as my truth, but made more so if you are dedicated to overthrowing society's paradigms became fashionable.
The most visibile, aggravating and dangerous (in my opinion) example is the emergence of Political Correctness. This seeks to subvert any cultural norms in favor of a feigned acceptance of any purported ism (feminism, multi-culturalism). One writer describing the phenomena states "...Like all academic foolishness, it (postmodernism) has an argot of jargon, tropes and incoherent phraseology recapitulated continuously by the cognoscenti. It distills, ultimately, to mere posturing as a substitute for intellectual fervor. Although nothing, according to the postmodernist, can be determined to be "true", postmodernism itself is, of course, True..."