Lambda abstraction

A lambda abstraction is an abstract 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: each its variables is bound by some lambda.
E.g.

f (f (f x))

is a concrete lambda expression, whereas

λf. λx. f (f (f x))

is an abstract lambda expression.