After reading the book, I feel that I understand what postmodernism is but there were so many descriptions that it became overwhelming and difficult to give a very brief definition of postmodernism.

As for me. postmodernism is a critic of modernism. On page 13, they say that it "rediscover the essence of humanity, finding an eternal value beyond all the chaos. I think this movement started by the industrial revolution to the time this book was written. Its summarizes  episodes in the course of history and critics the beliefs or facts that were inexplicable by science.  As for me there will always be a postmodernism movement because "modern" will come and a critic of it after it will come.