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In astronomy, an asterism is any pattern of stars recognized in the Earth's night sky. It may be part of an official constellation or it may be composed of stars from more than one constellation. Asterisms are composed of stars which, although visible in the same general area of the sky as viewed from Earth, are located at very different distances from Earth, at great distances from each other. Many asterisms are simple shapes composed of a few bright stars, making them easy to identify, and particularly useful to people who are familiarizing themselves with the night sky.

In colloquial usage, constellation is a synonym of asterism. But in astronomy, a constellation is an officially recognized area of the sky, surrounding certain asterisms. For example, the asterism known as the Big Dipper comprises the seven brightest stars in the International Astronomical Union (IAU) constellation Ursa Major. Asterism is the more general term, referring to any identified pattern of stars.

In many early civilizations, it was already common to associate groups of stars in connect-the-dots stick-figure patterns; some of the earliest records are those of the Babylonians. This process was essentially arbitrary, and different cultures have identified different constellations, although a few of the more obvious patterns tend to appear in the constellations of multiple cultures, such as those of Orion and Scorpius. Anyone could arrange and name a grouping, which could even overlap with other groupings, and since there was no "official" list, there was no difference between a constellation and an asterism.

An official list began to develop with Greco-Roman astronomer Claudius Ptolemy of Alexandria (c. 100 – c. 170), whose list of 48 constellations was accepted as standard in Europe for 1,800 years. As constellations were considered to be composed only of the stars that constituted the figure, it was always possible to use any leftover stars to create and squeeze in a new grouping among the established constellations. Furthermore, exploration by Europeans to other parts of the globe exposed them to stars unknown to ancient Mediterraneans. Two astronomers particularly known for expanding on Ptolemy's catalog were Johann Bayer (1572–1625) and Nicolas Louis de Lacaille (1713–1762). Bayer listed a dozen figures that had been suggested since Ptolemy's day; Lacaille created new groups, mostly for the area near the South Celestial Pole. Many of their proposed constellations have been accepted, with the rest remaining as asterisms. In 1930, the International Astronomical Union (IAU) divided the sky into 88 official constellations surrounding recognized asterisms, with precise geometric boundaries that encompass all of the stars within them. Any grouping – whether it has an area of the sky identified by it or not – is an asterism.

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