An asterism is a star-pattern that is not a constellation. Asterisms may be contained within a single constellation or consist of stars belonging to different constellations. The Big and Little Dippers are asterisms within the constellations of Ursus Major and Ursus Minor. The Summer Triangle and Winter Hexagon are asterisms that include stars from several constellations.
Related Note: "Asterism" derives from the Greek word for star, aster. However, in ancient astronomical texts aster was often used for a combination of stars rather than for single stars. Aster could also refer to a planet, or to a particular configuration of planets or a conjunction of planets and stars. This usage might be implicit in Matthew's description of the Star (aster) of Bethlehem which, if it was not an angelic or supernatural phenomenon, need not have been a single star.