Lambda calculus

Haskell theoretical foundations General: Mathematics - Category theory Research - Curry/Howard/Lambek Lambda calculus: Alpha conversion - Beta reduction Eta conversion - Lambda abstraction Other: Recursion - Combinatory logic Chaitin's construction - Turing machine Relational algebra

The lambda calculus is a formal mathematical system for expressing the notion of computation. Most functional programming languages are based upon the lambda calculus.

Wikipedia has more to say on the matter: Lambda calculus at Wikipedia.