Cultural appropriaton is the adoption, borrowing, or theft of elements created by or belonging to one culture or socially-coded identity by those belonging to a different one.
This is the most general and abstract sense, but the term is mainly used in situations where people with a more privileged identity borrow or steal from those with a less privileged identity. This is considered to be a human rights issue by many people, including me. The argument is that those with greater privilege have a greater control of mainstream culture, and can thus obscure the truth of cultural practices and motifs, as well as at times profiting unfairly from the appropriation in terms of credit and gain. See the article on Cultural Appropriation, Minstrel Shows, and African-American Music for a protracted examination of one example of this.
The most common examples of controversial cultural appropriation include things like white people wearing blackface, non-indigenous people using ceremonial headdresses as decorative accessories, male comedians dressing up as women and doing grotesque parodies of their idea of what women are, straight people pretending they are gay for comic effect, and so forth.
It is important, however, not to ignore other debatable types of cultural appropriation. For instance, recuperation is the action whereby mainstream mass media culture re-absorbs radical oppositional subcultures (such as the hippies, punk rock, or gangsta rap) and turns them into a defused and safe part of the mainstream spectacle, as characters in sit-coms or elements in advertising, etc.
Nowjacking® – a process where historically diverse cultures are made to seem consistent with or just like current mainstream culture – is another example of cultural appropriation.
See also postmodern decontextualization, travesty, the politics of self-representation, recuperation, nowjacking.