Omega (uppercase/lowercase Ω ω) is the 24th and the last letter of the Greek alphabet. It stood for a long "o" sound in Ancient Greek. It still stands for "o" in Modern Greek, but there is no longer a difference between long and short vowels, so it sounds the same as Omicron. In the system of Greek numerals, it stands for the number 800.
There was also once a letter "Omega" Ѡ in the Cyrillic alphabet. It was there so people could spell Greek names both in Cyrillic and in Greek in the same way. However, this letter is no longer used today.
In mathematics, the uppercase omega Ω is used in big-Omega notation (a variant of big-O notation). It is also used to represent the Omega constant. In electromagnetism and engineering, the uppercase Ω is used as the symbol for ohms, which are the units of electrical resistance. In physics and other sciences, the lowercase ω is often used to represent angular frequency.
In modern English, the phrase "From alpha to omega" expresses the idea of "everything".