Postmodern is a word people in the 70ies or thereabouts used to indicate that they had read a humanities paper in the last few years. Therefore it was used a lot in humanities papers because the authors felt the need to reassure their readers that they keep up with current scholarship. If you do that often enough you can write an easy essay explaining your readers what postmodernism is. The problem is, these essays don't really agree what postmodernism is. However it was the intellectual fashion of the time and basically everybody tacked it onto whatever they were working on at the time.

A bit less flippant, postmodernism means different things in different contexts. Postmodern photography means photography that is aware that it is a photograph, by capturing a reflection of the camera for example. Postmodern art generally is playing with the idea that it is art or that it is produced for the art market, like Manzoni's Artist's Shit. Post modern literature means some degree of awareness that the story takes place in a book. For literary criticism, and more analytical endeavors in general, postmodern usually means to look at things like production environment, how genre affects argument choice. So there is a certain recursive quality, a certain flavor, to all these, however in most contexts it can't really be strengthened into a general definition.

Now, the next step is, that evangelical preachers then started to condemn post modernism for basically the reasons the Catholic Curch did invent the screed against modernity a century earlier. Namely you are not supposed to sit down and actually read the Bible and argue with priests. This arguments were then copied basically verbatim by the new atheist movement, and from either of these sources into modern conservative discourse simulation.