Any of the 88 areas into which the celestial sphere is divided for the purposes of identifying objects, as adopted by the International Astronomical Union in 1922 (see Table 3, Appendix). In 1930 the IAU went on to adopt official constellation boundaries, defined by lines of right ascension and declination for the epoch 1875. This epoch was chosen because it had already been used by B. A. Gould to draw boundaries for the southern constellations. The IAU boundaries, defined by the Belgian astronomer Eugène Joseph Delporte (1882–1955), were published in Délimitation scientifique des constellations (1930) and the related Atlas céleste (1930).

The brightest stars in a constellation are identified by a Greek letter (the system of Bayer letters) or by a number (its Flamsteed number). When referring to stars identified in this way, the genitive case of the constellation’s name is always used, as in Alpha Orionis, 61 Cygni, or Zeta Ursae Majoris. Three-letter abbreviations of constellation names, as laid down by the IAU, are also frequently encountered (e.g. Ori, Cyg, or UMa).

The constellations officially recognized today are based on a group of 48 Greek figures listed by Ptolemy in the second century ad, with subsequent additions by various others. At the end of the sixteenth century two Dutch navigators, Pieter Dirkszoon Keyser (c.1540–96) and Frederick de Houtman (1571–1627), created twelve new constellations in the far southern part of the sky, below the horizon to Greek astronomers. The Dutch celestial cartographer Petrus Plancius (1552–1622), the Latinized form of Pieter Platevoet, added three more constellations in the spaces between those known to the Greeks, and he separated the stars of Crux from Centaurus.

The northern constellations that we know today were completed by J. Hevelius in 1687, who introduced several new figures, seven of which are still recognized today. In the 1750s the southern sky was filled out with fourteen new constellations by N. L. de Lacaille, who also split the large Greek figure Argo Navis into three parts.

http://www.iau.org/public/constellations/ International Astronomical Union official page on the constellations.

http://www.ianridpath.com/startales/contents.htm History and mythology of the constellations.