this is easily found on a google search:
http://www.google.com/search?hl=en&i...ism+definition
anyway, postmodernism relates to movements in the arts which reject the tenets of modernism as static, oppressive, heroic, and based on the values of the dominant culture.
postmodernism posits an alternative in which anti-heroic, and anti-dominant-culture attitudes are emphasized:
informality as opposed to formality
appropriation rather than authorship
non-linear as opposed to linear
multi-media rather than monomedia