Not a function: “one to many“. In other words, let’s say you had one x-value that maps to many y-values. For example, — in coordinate notation — (2,1) and (2,10). If the first number (the x-value) repeats, then you do not have a function. To put that another way, ...
How many degrees do condyloid joints move? What type of fibrous joint connects the tibia and fibula? What type of synovial joint is the tarsal-tarsal? What are condyloid joints made of? What two functional classes do most fibrous joints belong to?