A function f(x) assigns unique output values for every input value x. It is a relationship that associates each x value with exactly one y value.
The set of all possible input values (x-values) for which the function is defined.
The set of all possible output values (y-values) that the function can produce.
Vertical Line Test: A test to determine if a graph represents a valid function by checking if any vertical line intersects the graph at more than one point.