In the study of evolutionary relationships, derived refers to traits that exist in a present organism, but did not exist in the that organism's ancestors. For example, early primates were small and had raccoon-like snouts. Thus primates today which appear substantially different, such as the chimpanzee, would be considered derived.
The opposite of derived is "primitive," which refers to traits in modern organisms which existed in their ancestor group. Thus flight in modern birds would be considered primitive, as the earliest birds had flight, and flightlessness in modern birds would be considered a derived feature.