Postmodernism, in philosophy, is a term originally coined by Jean-François Lyotard in The Postmodern Condition. He defined it as the condition of society where people were generally skeptical of "metanarratives". A metanarrative, for Lyotard, is a grand narrative of all humanity which we are collectively working towards in some way. For example, that science will break down all truths to reductionist physics, or that a dialectical method will resolve the contradictions in society to a point where humanity can live in an ideal way. Lyotard was using the term descriptively, and in a negative sense. In other words, he did not advocate for postmodernism, but was merely describing a collapse in the belief in modernity. The term is often also closely associated with "post structuralism", and Jacques Derrida, and a sort of "irrationality". Like Lyotard, Derrida may have believed that what Enlightenment thinkers thought of as "Reason" wasn't as simple and straightforward as was previously believed, and that the term itself had subjective, historical, and cultural meaning imposed upon it by context in which it was used, so a narrative that we can easily move forward to being "governed by reason" was rooted in the dominate culture imposing their version of "reason". He certainly didn't believe this sort of analysis was "irrationality", and even less so advocated for abandoning reason itself, but advocated for a more careful analysis of what "reason" is. He did this, of course, through what he considered rational discourse.

Unlike these historical and philosophical definitions, the term "postmodernism" is used by many far right pundits as a sort of boogeyman. What they mostly mean by it is some sort of Marxism (who was a very modernism thinker, in that he thought the material dialectic was the Grand Narrative of history), or more vaguely, anyone who is at all to the left of them is an irrational "postmodernist". These people generally advocate for a return to "reason" or "rationality", which somehow always equates to more concentrated power in the hands of people exactly like themselves. They ignore, of course, that no one in history has advocated for being against reason, only for understanding what reason is, and how it operates in our culture today. Of course, they don't care about any of these philosophical debates, they just want to reverse the tide of feminism or whatever. Well, tough luck for them, because it isn't going to happen.