A term commonly used in functional programming languages to indicate a class of higher-order functions accepting other functions as inputs.

Specifically, from Combinators and Graph Reduction, Introduction (see below):

[..] Combinators are a special class of higher-order functions which were invented to study the elimination of bound variables from languages of logic, arithmetic and programming. By eliminating bound variables, we can study various formal properties of these languages independent of the usual problems associated with variable clashes and renamings. In particular, this leads to the possibility of an algebraic approach to programming and reasoning about programs, in which we can freely replace equals with equals in a direct, equational way. This contrasts with the usual approach to functional programming based on lambda calculus; [..]

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