Postmodernism means at least 5 different things, though they are not unrelated:

Step 1. A vague term for anything that comes after modernism - for instance architecture that no longer fits into what might be called "modern architecture";

Step 2. As Lyotard would have it, a condition characterized by incredulity towards metanarratives;

Step 3. As Jameson emphasizes, an aesthetics and a "cultural logic" of pastiche - which can be understood either as a multiplicity of partial and incompatible narratives, or as an ironic repetition, or both;

Step 4. An end to belief in nature;

Step 5. An end to belief in truth.