1590s, "a constellation, a group of stars," from Greek asterismos "a marking with stars," from aster "star" (see astro-). Any grouping of stars, whether a constellation or not (though in modern use, usually the latter). The "Big Dipper" is an asterism, not a constellation.
A conspicuous star pattern that is not recognized as a constellation. Asterisms may constitute a part of a larger constellation, as in the case of the seven stars in Ursa Major that make up the Big Dipper, or they may be formed of individual stars in several different constellations, as in the case of the Summer Triangle, made up of Deneb (in Cygnus), Altair (in Aquila), and Vega (in Lyra).