Inside jQuery we’ve found several situations where we need to know if an argument is numeric, or would be successfully converted to a number if it is some other type. We decided to write and document jQuery.isNumeric() since it’s a useful utility. Pass it an argument of any type and it returns true or false as appropriate.

jQuery.isNaN(): This undocumented utility function has been removed. It was confusing because it appropriated the name of a built-in JavaScript function but did not have the same semantics. The new jQuery.isNumeric() serves a similar purpose, but has the benefit of being documented and supported. Despite jQuery.isNaN() being undocumented, several projects on Github were using it. We have contacted them and asked that they use jQuery.isNumeric() or some other solution.

Also see the ticket: http://bugs.jquery.com/ticket/10478

jQuery's isNumeric() checks if a value is a number OR can be converted to a number.

EDIT

To further clarify what isNan() does (and what a NaN value is):

A NaN, which means Not-a-Number, is classified as a primitive value by the ECMA-262 standard and indicates that the specified value is not a legal number. The function returns true if the argument is not a number and false if the argument is a number.

The classic example of a NaN is zero divided by zero, 0/0

Code: 

document.write(isNaN(Ima String)) 

document.write(isNaN(0/0)) 

document.write(isNaN(348)) 

document.write(isNaN(348)) 



Output: 

true 

true

false

false

http://www.devguru.com/technologies/ecmascript/quickref/isnan.html

Semi offtopic, but related is this short talk