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

bracket abstraction

    An algorithm which turns a term into a function
   of some variable.  The result of using bracket abstraction on
   T with respect to variable v, written as [v]T, is a term
   containing no occurrences of v and denoting a function f such
   that f v = T.  This defines the function f = (\ v . T).  Using
   bracket abstraction and currying we can define a language
   without bound variables in which the only operation is
   monadic function application.

   See combinator.

   (1995-03-07)