There are 24 letters in the Greek alphabet and Omega(Ω) is one of them. Omega(lowercase ω, uppercase Ω) is the 24th and final letter of the Greek alphabet and has a value of 800 in the Greek numerals system. It is written in the shape of a circle. The letter itself is called the final letter of the Greek alphabet because it is the last letter of the Greek alphabet, and its use as a separate character is relatively recent.

Source: Symbols and Meanings