Is it ensure, insure, or assure?
A program transformation to remove free variables. An expression containing a free variable is replaced by a function applied to that variable. E.g.
f x = g 3 where g y = y + x
x is a free variable of g so it is added as an extra argument:
f x = g 3 x where g y x = y + x
Functions like this with no free variables are known as supercombinators and are traditionally given upper-case names beginning with "$". This transformation tends to produce many supercombinators of the form f x = g x which can be eliminated by eta reduction and substitution. Changing the order of the parameters may also allow more optimisations. References to global (top-level) constants and functions are not transformed to function parameters though they are technically free variables.
A closely related technique is closure conversion. See also Full laziness.