In mathematics, "undefined" refers to a situation where a value cannot be determined or does not have a meaningful interpretation. For example, dividing any number by zero, such as 5/0, is considered undefined because there is no number that can satisfy the equation. This concept is crucial in understanding limits and functions in calculus. In programming, "undefined" often indicates that a variable has been declared but not assigned a value. For instance, in languages like JavaScript, if you create a variable without initializing it, its value is "undefined." This helps developers identify errors and manage data more effectively.