What is Post Modernism in philosophy?

I do not believe there is a definition of Postmodernism.It is more like a series of concepts that diifferentiates another series of concepts.It reflected differences of view in art, literature, history, and culture.Its period of usage seems to have been the 1950s through the 1970s.It first appesred in the Philosophical Dictionary in a publication by Jean-Francois Lyotard called "The Postmodern Condition".The Post doesn't appear to refer to after Modernism but different from Mocernism in the aspects of the Human Condition I mentioned.You will find explanations in the publications from various universities, but never a clear cut definition.Read all of the Journals and Encyclopedias and all you will find is how Postmodernism has affected these various human endeavors.(acouple of sentences earlier I typed Mocernism when I meant Modernism).I believe it would be a mistake to describe Postmodernism as a separate School of Thought but rather as an adjective to help understand a work of art,lierature or music. You have to please excuse my keyboarding as I must be the worst.I only hope you are able to read all that I type.I have been studying philosophy for 35 years and I have come to no closer to a definite answer than you, but I will continue the effort.