Gamma (upper case Γ, lower case γ) is the third letter of the Greek alphabet. In the system of Greek numerals it has a value of 3.

It gives its name to several things:

• Gamma characteristic power law, and gamma correction in television and computer display systems. Gamma is the exponent in a power-law relationship between video or pixel values and the displayed brightness.
• In the game of Tetris, gamma is another name for the J tetromino.
• 1 gamma is a unit of magnetic flux density, 1 gamma = 10^-9 tesla or 1 nanotesla.

The upper-case letter Γ is used as the symbol for:

• The gamma function (related to factorials) in mathematics.
• The Gamma distribution in probability theory and statistics.
• The symbol for Γ is used on the hat of the videogame character Waluigi
• The reflection coefficient in electrical engineering.

Gamma is the third caste in Aldous Huxley's dystopic satire Brave New World.

The lower-case letter γ is used as the symbol for:

• The Euler-Mascheroni constant in mathematics.
• A risk management measure in mathematical finance.
• The photon, seen as an elementary particle in physics.
• Gamma rays in physics and astronomy.
• The gamma factor in relativity theory and astronomy.
• The adiabatic index of a gas in physics. Often the letter kappa κ is used.
• The propagation constant of an electromagnetic wave (in electrical engineering and electromagnetic theory).

Letters that arose from Gamma are the Roman C and G and the Cyrillic letter Ghe.