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

beta abstraction

   [lambda-calculus] The conversion of an expression to an
   application of a lambda abstraction to an argument
   expression.  Some subterm of the original expression becomes
   the argument of the abstraction and the rest becomes its
   body. E.g.

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

   The opposite of beta abstraction is beta reduction.  These
   are the two kinds of beta conversion.