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 ( ), whereas the Omicron was called ou ( ). 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 1, similar to the vowel of English raw in certain dialects such as RP where this word is pronounced 2. It should not be confused with the sound of English so-called "long o" as in ocean, which is either diphthongal 3 or 4 or a close-mid kind of "o" 5, in either case distinctly different from the "long o" sound of Greek omega and closer to the Classical Greek sound represented by ου before this came to be pronounced 6. 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, God is declared to be the "alpha and omega, the beginning and the end, the first and the last".

As Ninhursag's symbol, 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 ( , ). A Raetic variant is at the origin of Elder Futhark ᛟ.

The symbol Ω (majuscule letter)

The upper-case letter Ω is used as a symbol. It is also used often outside its Greek alphabetical context in literature, advertising and other forms of human expression.

• 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.
• The solid angle or the rate of precession in a gyroscope.
• In particle physics to represent the Omega baryons.
• 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 (also known as ω₁)
• In combinatory logic, the looping combinator, (λ x. x x) (λ x. x x)
• In statistics, it is used as the symbol for the sample space, or total set of possible outcomes.
• In astronomy (cosmology) Ω refers to the density of the universe, also called the density parameter.
• The symbol of the Resistance Movement against the Vietnam-era draft
• Year or date of death
• In God of War (video game), Incorporated into the logo design.
• The logo of Omega Watches SA.
• In Warhammer 40,000, the Space Marine chapter "Ultramarines" uses an upside-down variation of it.
• Omega Energy Drink made by fuze uses the symbol on the can.
• The 1978 comedy Animal House portrays a college fraternity called Omega, which seeks to undermine the crazy antics of the neighboring Delta Tau Chi fraternity.
• In the Red vs. Blue series, Omega is an Artificial Intelligence implanted into the armor of Freelancer Texas (Tex), but then goes insane and tries the rule the universe.