Crescent

In art and symbolism, a crescent 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). 

The crescent was long used as the symbol of Byzantium. Now it is the main symbol for the religion Islam.

 Other websites 

The origin of the Islamic crescent
MathWorld article on geometric "Lune"
Worldwide Moon Sighting Reports 

Category:Symbols
Category:Religious symbols
Category:Moons