Omega (majuscule: Ω, minuscule: ω; Greek Ωμέγα) is the 24th and last letter of the Greek alphabet. In the Greek numeric system, it has a value of 800. The word literally means "great O" (ō mega, mega meaning 'great'), as opposed to Omicron, which means "little O" (o mikron, micron meaning "little"). This name is Byzantine; in Classical Greek, the letter was called ō (ὦ), whereas the Omicron was called ou (οὖ). The form of the lowercase letter derives from a double omicron, which came to be written open at the top.

Phonetically, the Ancient Greek Ω is a long open-mid o [ɔː], similar to the vowel of English raw as pronounced in New York. In Modern Greek Ω represents the same sound as omicron. The letter omega is transcribed ō or simply o.

Omega (the last letter of the Greek alphabet) is often used to denote the last, the end, or the ultimate limit of a set, in contrast to Alpha, the first letter of the Greek alphabet. In the New Testament book of Revelation, Christ is declared to be the "alpha and omega, the beginning and the end, the first and the last".

As the symbol of the Sumerian goddess Ninhursag, the omega has been depicted in art from around 3000 BC, though more generally from the early second millennium. It appears on some boundary stones on the upper tier, which indicates her importance.

Omega was also adopted into the early Cyrillic alphabet. See Cyrillic omega (Ѡ, ѡ). A Raetic variant is conjectured to be at the origin or parallel evolution of the Elder Futhark ᛟ.

• In physics:
  • For ohm — SI unit of electrical resistance; formerly also used upside down (℧) to represent mho, the old name for the inverse of an ohm (now siemens with symbol S) used for electrical conductance. Unicode has a separate code point for the ohm sign (U+2126, Ω), but it is only included for backwards compatibility and the Greek uppercase omega character (U+03A9, Ω) is preferred.
  • For angular velocity in circular motion
  • In statistical mechanics Ω refers to the multiplicity (number of microstates) in a system.
  • The solid angle or the rate of precession in a gyroscope.
  • In particle physics to represent the Omega baryons.
  • In astronomy (cosmology) Ω refers to the density of the universe, also called the density parameter.
• In mathematics/computer science:
  • in notation related to Big O notation to describe the asymptotic behavior of functions.
  • Chaitin's constant.
• In mathematics
  • In complex analysis, the Omega constant, a solution of Lambert's W function
  • A variable for a 2-dimensional region in calculus, usually corresponding to the domain of a double integral.
  • In set theory, the first uncountable ordinal number (more commonly written as ω₁)
  • In topos theory, the (codomain of the) subobject classifier of an elementary topos.
  • In combinatory logic, the looping combinator, (λ x. x x) (λ x. x x)
  • In group theory, the omega and agemo subgroups of a p-group, Ω(G) and ℧(G)
  • In statistics, it is used as the symbol for the sample space, or total set of possible outcomes.
• As part of logo or trademark:
  • The logo of Omega Watches SA.
  • Part of the Badge of the Supreme Court of the United Kingdom.

• In fictional worlds:
  • In the fictional world of Star Trek, the Omega symbol is used in conjunction with the Omega Directive.
  • As the logo of the popular PlayStation franchise God of War.
  • In the fictional universe Warhammer 40,000, the symbol for the Sons of Orar Chapter and, if turned upside down, the symbol for the Ultramarine Chapter of the Space Marines (Warhammer 40,000).
  • In the popular web series Red vs Blue the Blood Gulch Chronicles Omega (or o'mally) is freelancer Tex's AI implant and the main antagonist of the Blood Gulch Chronicles

• Other
  • The Marilyn Manson album Mechanical Animals has false band within the album called Omēga and the Mechanical Animals
  • The symbol of the resistance movement against the Vietnam-era draft
  • Year or date of death
  • Used to refer to the lowest-ranked wolf in a pack