beta reduction

Noun

 * 1) The act of beta reducing, an instance of replacing a function call by the result of calling a function.
 * 2)  One of the three rewrite rules of the lambda calculus, which states that the application of a lambda abstraction $$(\lambda x. t) $$ to a term $$s$$ yields an expression $$t[s/x]$$, "t with s instead of x", where all free instances of x in t have been replaced by s.