A crescent is the shape you get if you cut out of a circle another circle of roughly the same size, but offset to one side. You might also describe it as the shape the lit side of a sphere takes on if, in respect to the viewer, the light source is behind and to one side of the sphere (as in a crescent moon).

The word Crescent appears in the name of some places:

• Crescent City, California

The crescent is also often used to symbolize the Islamic faith. However, it should be noted that the crescent was not a symbol used for Islam by Muhammad or any other early Muslim rulers, in fact the Islam is against appointing "Holy Symbols". Despite that the 5-branch star is quite often used as the symbol of the Islam.

Many Christian fundamentalists such as Jack Chick use this symbolism to make a claim that Allah was in fact a moon-god worshipped by the pre-Islamic Arabs. This argument is sometimes used to argue that the god of Islam is different from that of the Jews or Christians.

A discussion of competing theories about the origin of the crescent as an Islamic symbol can be found here: http://www.fotw.net/flags/islam.html#ori.