In

lambda calculus,

Lisp,

scheme, and

haskell, lambda is used
to create an

abstraction---that is, an unnamed function. For
example, the function which computes the square of its argument
may be written as:

*\lambda x . * x x* (in lambda calculus, assuming * has been
defined)

`(lambda (x) (* x x))` (in Lisp and scheme)

`\ x -> (x * x)` (in haskell).

In

ML, lambda is called `

fn':

`fn x => (x * x)`