asterism

1. (astronomy) An unofficial constellation (small group of stars that forms a visible pattern).Examples: "The Big Dipper, Summer Triangle, and Orion's Belt are asterisms."; "Of the constellations, or asterisms. [...] It therefore became necessary to adopt a more general method of distinguishing them. This was accomplished by portioning out the heavens into imaginary figures, of men, birds, fishes, &c. called Constellations or Asterisms."; "Like Pisces and Capricorn, Aquarius is located in that part of the sky known as "Astral Sea," a region inhabited by water asterisms such as Eridanus, Cetus (the Whale), and the Dolphin."

2. (mineralogy) A star-shaped figure exhibited by some crystals by reflected light (as in a star sapphire) or by transmitted light (as in some mica).