Combinatory logic (Curry and Feys 1958) is a “variable-free” alternative to the lambda
calculus. The two have the same expressive power but build their expressions
differently. “Variable-free” semantics is, more precisely, “free of variable binding”: it
has no operation like abstraction that turns a free variable into a bound one; it uses
combinators—operations on functions—instead.

http://homepages.nyu.edu/~as109/szabolcsi,_binding_on_the_fly,_2003.pdf