The word "constellation" comes from the Late Latin term cōnstellātiō, which can be translated as "set of stars"; it came into use in English during the 14th century. The Ancient Greek word for constellation is ἄστρον. A more modern astronomical sense of the term "constellation" is simply as a recognisable pattern of stars whose appearance is associated with mythological characters or creatures, or earthbound animals, or objects. It can also specifically denote the officially recognized 88 named constellations used today.
Colloquial usage does not draw a sharp distinction between "constellations" and smaller "asterisms" (pattern of stars), yet the modern accepted astronomical constellations employ such a distinction. E.g., the Pleiades and the Hyades are both asterisms, and each lies within the boundaries of the constellation of Taurus. Another example is the popular northern asterism known as the Big Dipper (US) or the Plough (UK), composed of the seven brightest stars within the area of the IAU-defined constellation of Ursa Major. The southern False Cross asterism includes portions of the constellations Carina and Vela.
A constellation (or star), viewed from a particular latitude on Earth, that never sets below the horizon is termed circumpolar. From the North Pole or South Pole, all constellations south or north of the celestial equator are circumpolar. Depending on the definition, equatorial constellations may include those that lie between declinations 45° north and 45° south, or those that pass through the declination range of the ecliptic or zodiac ranging between 23½° north, the celestial equator, and 23½° south.
Although stars in constellations appear near each other in the sky, they usually lie at a variety of distances away from the Earth. Since stars have their own independent motions, all constellations will change slowly over time. After tens to hundreds of thousands of years, familiar outlines will generally become unrecognizable. Astronomers can predict the past or future constellation outlines by measuring individual stars' common proper motions or cpm by accurate astrometry and their radial velocities by astronomical spectroscopy.