Static scoping
With static scoping, a variable always refers to its nearest enclosing binding.
Dynamic scoping
In dynamic scoping, each identifier has a global stack of bindings. Introducing a local variable with name x pushes a binding onto the global x stack (which may have been empty), which is popped off when the control flow leaves the scope. Evaluating x in any context always yields the top binding. Note that this cannot be done at compile time because the binding stack only exists at runtime, which is why this type of scoping is called dynamic scoping.