"Not a Number" (often represented as "NaN") is a term used to describe a value that is undefined or unrepresentable in a numerical system. It is commonly used in computing to represent the result of an undefined or unrepresentable mathematical operation, such as the square root of a negative number or the division of zero by zero. NaN values are often used to indicate an error or missing data in a computation, and they can also be used to propagate error values through a calculation.
W3Schools - NaN (Not a Number)
MDN Web Docs - isNaN()