Merriam-Webster Collegiate Dictionary - omega

 noun  Etymology: Middle English, from Greek o mega, literally, large o  Date: 15th century  1. the 24th and last letter of the Greek alphabet — see alphabet table  2. the extreme or final part ; end reached the ~ of his career  3.  a. a negatively charged elementary particle that has a mass 3270 times the mass of an electron — called also ~ minus  b. a very short-lived unstable meson with mass 1532 times the mass of an electron — called also ~ meson