A closure is a persistent local variable scope

Languages which support closure (such as JavaScript, Swift, and Ruby) will allow you to keep a reference to a scope (including its parent scopes), even after the block in which those variables were declared has finished executing, provided you keep a reference to that block or function somewhere.

outer = function() {
  var a = 1;
  var inner = function() {
    console.log(a);
  }
  return inner; // this returns a function
}

var fnc = outer(); // execute outer to get inner 
fnc();

As you might be able to guess, when I call fnc() it prints the value of a, which is "1".

===========================

function add (a) {
  return function (b) {
    return a + b;
  }
}

var add3 = add(3);

add3(4); // returns 7
========================
var add = (function () {
  var counter = 0;
  return function () {counter += 1; return counter}
})();

add();
add();
add();

// the counter is now 3