The term 'functor' has two common meanings:
1. Function object. In object-oriented languages, it is a feature that allows objects to be used like if they were ordinary functions.
2. A mathematical structure which deals with mappings between categories. This concept is a useful abstraction in some programming languages, notably Haskell, where it is implemented as a type class.
3. In OCaml, a module that takes another module as an argument.
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