| In the world of art and cultural studies, the word appropriation has two related but semi-conflicting meanings, one with negative and one with positive connotations.
First the positive: "Appropriation" in the art world refers to the practice of producing art out of other art works or cultural artifacts. A whole group of young artists in the 80s became notorious for this, including Jeff Koons, Sherrie Levine, and Cindy Sherman, but the general practice is at least as old, in fine art, as the first collages of Picasso and Braques (and even older if you count non-western and non-academic cultural practice).
Other visual artists that provided important milestones in this area include Andy Warhol, Robert Rauschenberg, and Bruce Conner. This happens in other art forms as well -- in music (especially since the advent of digital sampling with artists ranging from John Cage to Negativland to Pop Will Eat Itself to Public Enemy.
Many critics of the technique call it simply plagiarism (though audio artists like The Tape-beatles used that word as a positive name for their own creative process). John Oswald coined the term "plunderphonics" to describe his exquisite sample-based music.
Other terms include culture jamming, detournment, and my personal favorite, recycled culture (I have a web site that's all about this sort of thing, but I guess I'm not allowed to link to it from here).
Needless to say intellectual property law often makes this kind of art difficult to do.
Speaking of critics, on the negative connotation side, appropriation is used in cultural studies, anthropology, ethnomusicology, and related fields to describe the act of one people coopting culture from another - usually Westerners stealing cultural ideas from people in developing nations. Examples abound: whites wearing bindis, The Police appropriating reggae, Madonna turning Indian on her "Ray of Light" album, and the so-called "primitivist" movement in fine art where European artists like Paul Klee and Jean Dubuffet copied the look of prehistoric or tribal art as a way of revitalizing their own dried-up academic tradition.
We also see this negative appropriation in recent discussions of advertising, as the marketeers increasingly scoop up any bit of culture they can find that might help sell things -- capitalism appropriating culture, such as Nike using The Beatles song "Revolution" to sell shoes.
Of course this sort of thing is going to become increasingly important and contested as we move further and further into the information age, where ideas and signs are increasingly commodities. Appropriation is truly part of the post-modern condition. | 
