Several defenitions:
- closure is when a function is able to remember and access its lexical scope even when that function is executing outside its lexical scope.
- a closure is one way of supporting first-class functions; it is an expression that can reference variables within its scope (when it was first declared), assigned to a variable, passed as an argument to a function, or returned as a function result.
- a closure is a stack frame which is allocated when a function starts its execution, and not freed after the function returns (as if a 'stack frame' were allocated on the heap rather than the stack!).
Used in module pattern. Can be used for currying.