An outtake is any scene filmed for a movie that isn't used in the final cut. If an actor forgets his lines, for example, that scene will probably end up as an outtake.
Making a film is a long process that involves many steps even after the actual filming of actors reciting dialog and acting out scenes is long over. Editors cut unnecessary bits out of movies, discarding outtakes and shaping a final product. Occasionally these outtakes are interesting or funny enough that directors will include them as extras on a DVD of a movie. Outtake dates from about 1960.