Definition: NaN is a global property (variable) with a constant value. NaN is an abbreviation for "Not a Number". Anywhere that you call JavaScript processing that requires a number and you give it something that isn't a number the code will substitute NaN for the result. You can't compare a field with NaN in order to test whether a value is a number or not since anything that isn't set specifically to NaN will not be equal to it, you use isNaN() to test that instead. This is not a reserved word so you can declare your own variable or function called NaN but if you do then you will break the JavaScript functionality that determines whether something is or isn't a number.
Examples:
addOne = function(a) {return a++;}alert(addOne('seven'));