I forget all my vocabulary so hopefully someone else can correct me if the words I use aren't correct. But the following is definitely a function. "Although two x values correspond to a singular y value, the function passes a vertical line test. Meaning that it is ok to have values of x that equal the same y values, however two values of y assigned to the same x value would make the function undefined."
Hope this helps! The only vocabulary in this that you would really need is "corresponding," and (if you learned about it, you may not have yet) "vertical line test"