Vertical line test
In mathematics, the vertical line test is a way to determine if a curve is a graph of a function or not. A function can only have one output, y, for each unique input, x. If a vertical line intersects a curve on an xy-plane more than once then for one value of x the curve has more than one value of y so the curve does not represent a function. If a vertical line intersects a curve only once then the curve represents a function.
To use the vertical line test, take a rule or other straight edge and draw a line parallel to the y-axis for any chosen value of x. If the vertical line you drew intersects the graph more than once for any value of x then the graph is not a function. If, alternatively, the vertical line intersects the graph no more than once at any given point then the graph is a function. For example, any straight line other than a vertical line will be a function. As another example, the sideways parabola is not a function because a vertical line will intersect the parabola twice.