Can anyone provide a comprehensive definition of postmodernism?

Postmodernism is, by its nature, a reactionary ideology that seeks to reject the tenets of Modernism.

The core of Modernism itself is that truth is derived from effort. It doesn't matter what the truth is or whether you like it, but by conscious and applied work, there is some of fundamental, natural truth that can be discerned.

Postmodernism is the rejection of this notion - not only will effort not result in enlightenment about any fundamental truths, in fact there are no fundamental truths to be discovered in the first place. This is extended to the common refrain that "nothing is objective and everything is subjective": when there is nothing inherently good, then what is good must necessarily be contingent upon a person's view point because there is no other foundation for it to rest.

As for existentialism, the feeling of absurdity about the inherent meaningless of life is a natural follow-on from the postmodernist view that there is no inherent truth to be found. If there are no indisputable, natural truth at all, that must include a lack of purpose ascribed to someone in a cosmic sense.

FWIW I think most existentialists actually miss the trees for the forest at their despair over a perceived lack of cosmic importance. Even if something doesn't have universal meaning, it can still obviously have personal meaning. The interactions and relationships with others we have on a daily basis are absolutely real in the most real sense. In Sartre's Nausea, Roquentin even has a moment of lucidity upon hearing music playing from a record, as he loses his cosmic absurdity in the moment.