Lambda abstraction

A lambda abstraction is a lambda expression. It is expressed in the language of lambda calculus. A lambda abstraction is to a functional programming language such as Scheme what pseudo-code is to an imperative programming language.

More specifically, a lambda abstraction is a lambda expression with no free variables: all its variables are bound by some lambda.
E.g.

'f' ('f' ('f' 'x'))

is a concrete expression, whereas

λ'f'. λ'x'. 'f' ('f' ('f' 'x'))

is an abstract lambda expression.