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.