[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.