Part of the challenge is that the nature of postmodernism makes defining it very difficult. Any intellectual or pseudo-intellectual who comes at life or "truth" or philosophy or art or whatever from a postmodernist perspective will likely critique any definition we offer of postmodernism. It makes it a very slippery and dangerous thing.

To very much oversimplify, postmodernism is a mid-to-late 20th century movement that challenged modernity. It's a school of thought or approach that calls into question pretty much all "grand narratives," traditions, or truth claims. People within the postmodern school of thought love to "deconstruct" things. And they are often fans of moral relativism.

Today, postmodernism has given way to kind of a tribal or group-oriented relavitism wherewith everything is evaluated according to power dynamics between identity groups.

It's a real mess.