Closure : Function bundled together with its lexical environment/scope.

JS is a weird language. You can pass functions as parameters to another function, assign a variable to an entire function, or even return a function.
eg:

function x() {
  var a = 7;
  function y() {
  console.log(a);
}
  return y;   // instead of y();
}
var z = x();
console.log(z);  // value of z is entire code of function y.

When functions are returned from another fun, they still maintain their lexical scope.

• When y is returned, not only is the fun returned but the entire closure (fun y + its lexical scope) is returned and put inside z. So when z is used somewhere else in program, it still remembers var a inside x()
• Closure is a very powerful concept of JS, just because this function remembers things even if they are not in their lexical scope

Module Design Pattern, Currying, Functions like once(fun that can be run only once), memoize, maintaining state in async world, setTimeout, iterators...