The term lurks behind the received opinion of David Salle's paintings of the 1980s. As a noun in general, anything free of dependence upon factors external to itself. In metaphysical idealism specifically, the totality of what in fact exists. As an adjective in formal terminology, it has connotations related to the former. In politics, unrestricted political power.
Postmodernism generally rejects all forms of absolutism.
Compare, however, Wassily Kandinsky's conception of abstraction with that of contemporary artists like Peter Halley.