In the lambda calculus a *bound variable* is one that is
bound by a λ abstractor.
A variable that is not bound is called free.

`λx.(xy)`

- here`x`

is bound but`y`

is free.`λx.λy.(y(xz))`

- here`z`

is the only free variable and the rest are bound.

H. P. Barendregt. *The Lambda Calculus. Its Syntax and Semantics.* Elsiever, 1984.

Copyright © 2014 Barry Watson. All rights reserved.