Mathematically inclined friends

I have a vocabulary gap I need filling:

What is the name for a continuous curve which has exactly one y value for every x; or a surface which has exactly one z value for every (x,y); or etc? There's got to be a word to distinguish those from the ones that loop under themselves but I either never learned it or have forgotten it.

("I can't google it because I don't know its name" is the new "I can't find it in the dictionary because I don't know how to spell it")