In art and symbolism, a crescent (US: /ˈkrɛsənt/, UK: /ˈkrɛzənt/) is generally the shape produced when a circular disk has a segment of another circle removed from its edge, so that what remains is a shape enclosed by two circular arcs of different diameters which intersect at two points (usually in such a manner that the enclosed shape does not include the center of the original circle).
In astronomy, a crescent is the shape of the lit side of a spherical body (most notably the Moon) that appears to be less than half illuminated by the Sun as seen by the viewer. Mathematically, assuming the terminator lies on a great circle, such a crescent will actually be the figure bounded by a half-ellipse and a half-circle, with the major axis of the ellipse coinciding with a diameter of the semicircle. The direction in which the "horns" (the points at the intersection of the two arcs) face indicates whether a crescent is waxing (also young, or increasing) or waning (also old, or decreasing). Eastward pointing horns (pointing to the left, as seen from the Northern hemisphere) indicate a waxing crescent, whereas westward pointing horns (pointing to the right, as seen from the Northern hemisphere) indicate a waning crescent. Note that the directions the horns point relative to the observer are reversed in the Southern hemisphere.
The word crescent is derived etymologically from the present participle of the Latin verb crescere "to grow", thus meaning "waxing" or "increasing", and so was originally applied to the form of the waxing moon (luna crescens). The English word is now commonly used to refer to either the waxing or waning shape. In the technical language of blazoning used in heraldry, the word "increscent" refers to a crescent shape with its horns to the left, and "decrescent" refers to one with its horns to the right.