NaN, or "Not-A-Number," is a term used in computing to represent a value that does not represent a real number. It often occurs in programming and data analysis when calculations produce undefined or unrepresentable results, such as dividing zero by zero or taking the square root of a negative number. NaN is a standard part of the IEEE 754 floating-point specification. In many programming languages, including Python, JavaScript, and Java, NaN is used to handle errors in numerical computations gracefully. It allows developers to identify and manage invalid data without crashing the program. NaN is distinct from other values like null or undefined, which have different meanings in programming contexts.