The Free On-line Dictionary of Computing (30 December 2018):

beta reduction [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.