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:
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.
The flag of Turkey is similar to the flag used in late periods of Ottoman Empire.
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.
