What is the definition of constellations?

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Simply put, constellations are arbitrary groupings of stars that have been constructed so as to assist in the location and identification of stars. They don't have any real existence as separate coherent objects, apart from the way the groups of stars appear to observers on earth. It looks to us as if the stars of a constellation must be 'grouped' more closely together in space, simply because they appear together in the sky. In reality, a given star in a constellation may be extremely far away from the other stars in its constellation, and that star may in fact be closer to stars in other constellations than it is to stars in its own.

Constellations are really a group of stars that form a fixed patter in the night sky or pictures as the first person said.

A constellation is a group of stars that appear to have a physical proximity in the sky. The stars in a constellation are often vastly distant from each other, but they appear close to each other from the perspective of Earth. The word is used colloquially to refer to asterisms: groups of stars that appear to form patterns in the sky; different world cultures have divided the stars into different constellations. However, in modern astronomy the word refers instead to a method of dividing the sky into 88 areas with exact boundaries.