**The Free On-line Dictionary of Computing (30 December 2018):**

lambda-calculus
(Normally written with a Greek letter lambda).
A branch of mathematical logic developed by Alonzo Church in
the late 1930s and early 1940s, dealing with the application
of functions to their arguments. The pure lambda-calculus
contains no constants - neither numbers nor mathematical
functions such as plus - and is untyped. It consists only of
lambda abstractions (functions), variables and applications
of one function to another. All entities must therefore be
represented as functions. For example, the natural number N
can be represented as the function which applies its first
argument to its second N times (Church integer N).
Church invented lambda-calculus in order to set up a
foundational project restricting mathematics to quantities
with "effective procedures". Unfortunately, the resulting
system admits Russell's paradox in a particularly nasty way;
Church couldn't see any way to get rid of it, and gave the
project up.
Most functional programming languages are equivalent to
lambda-calculus extended with constants and types. Lisp
uses a variant of lambda notation for defining functions but
only its purely functional subset is really equivalent to
lambda-calculus.
See reduction.
(1995-04-13)