What are not functions in math?

In mathematics, a relation is not considered a function if it assigns multiple outputs to a single input. For example, the relation that maps a number to both its positive and negative square roots (like 4 to both 2 and -2) fails the vertical line test, which states that if a vertical line intersects a graph at more than one point, then the relation is not a function. Additionally, any relation that does not maintain a consistent output for every input is also not classified as a function.