Lambda Calculus: The Foundation of Functional Programming

Functional programming is based on a theoretical foundation called lambda calculus (also written as $\lambda$-calculus) , invented by Alonzo Church. The easiest way to understand lambda calculus is to think of it as an idealized bare-bones functional programming language. In other words, take OCaml or any similar functional programming language and strip down all features that do not contribute to the ability to do functional programming. What remains is essentially lambda calculus.