Non-local variable

In programming language theory, a non-local variable is a variable that is not defined in the local scope. While the term can refer to global variables, it is primarily used in the context of nested and anonymous functions where some variables can be neither in the local nor the global scope.

In Lua they are called the upvalues of the function.

In the Python 3 example that follows there is a nested function inner defined in the scope of another function outer. The variable x is local to outer, but non-local to inner (nor is it global):

In Javascript, the locality of a variable is determined by the closest var statement for this variable. In the following example, x is local to outer as it contains a var x statement, while inner doesn't. Therefore, x is non-local to inner:

In the Haskell example that follows the variable c is non-local in the anonymous function \x -> x + c:

Non-local variables are the primary reason it is difficult to support nested, anonymous, higher-order and thereby first-class functions in a programming language.

If the nested function or functions are (mutually) recursive, it becomes hard for the compiler to know exactly where on the call stack the non-local variable was allocated, as the frame pointer only points to the local variable of the nested function itself and there can be an arbitrary number of activation records on the stack in between. This is generally solved using access links or display registers.

...
Wikipedia