Closures are programming language constructs that include data alongside instructions that are not directly related to their input (arguments) and their results (return values). They can be seen as a “generalization” of the concept of a function or function call, in that a function call is a “subset” of closures (e.g., the set of closures that do not include this extra, spicy data that comes from places outside of arguments and returns). - The Cost of a Closure in C