Lambda

Lambda (Λ λ) is the 11th letter of the Greek alphabet and has a numeric value of 30.

The upper-case letter Λ is used as a symbol for:

The lower-case letter λ is used as a symbol for:

The wavelength of a wave in physics.

The Lambda phage virus in biology.

The definition of anonymous functions in lambda calculus and, by extension, some programming languages.

The language Unlambda is made up completely of lambdas, but they cannot be stored, hence the un-.