What do astronomers mean by a constellation?
A pattern created by linking certain stars, to form a composite figure, that resonates with a mythological figure, or some other recognizable pattern from common experience.
Constellations have been identified by the ancient Greeks, Indians, Babylonians, Egyptians etc., to serve, sometimes indirectly, the basis of astrology,
Constellations have different names in different cultures and were used to form patterns that observers could identify with the folklore of the region. eg) Pisces in western nomenclature is called Matsya in Indian nomenclature. Those constellations that lie along the ecliptic are used as the basis of astrology.
Famous constellations are Orion, the Great Bear, the Big Dipper etc. Constellations visible to the naked eye in the Northern hemisphere differ from those visible in the Southern hemisphere.