[lambda-calculus] The application of a lambda abstraction to an argument expression. A copy of the body of the lambda abstraction is made and occurrences of the bound variable being replaced by the argument. E.g.

        (\ x . x+1) 4  -->  4+1

Beta reduction is the only kind of reduction in the pure lambda-calculus. The opposite of beta reduction is beta abstraction. These are the two kinds of beta conversion.

See also name capture.