NaN stands for?

Answer 1

Using Google is sometime better to avoid simple and small questions. It simply means not a number

Answer 2

not a number

Answer 3

Sonu Kumar Kushwaha
There are some mathematical operations which yield results that are not real numbers. The answers may be made up or imaginary numbers or undefined or unknown values, either of which cannot be represented within a floating-point structure. In these situations, the result of the value will be NaN.

Answer 4

Not a number