In Java, what does NaN mean?

NaN means “Not a Number” and is basically a representation of a special floating point value in the IEE 754 floating point standard. NaN generally means that the value is something that cannot be expressed with a valid floating point number.

A conversion will result in this value, when the value being converted is something else, for example when converting a string that does not represent a number.

Taken from this page:

"NaN" stands for "not a number". "Nan" is produced if a floating point operation has some input parameters that cause the operation to produce some undefined result. For example, 0.0 divided by 0.0 is arithmetically undefined. Taking the square root of a negative number is also undefined.