all the others have for at least 1 value of x more than one different y values defined.
but a function must be clear about the association between an x unit value and the corresponding result.
many different input values can have the same functional result. but there can be only one defined result per input value. otherwise it is not a function.
you cannot say 1+1 = 2 or 3. it is either 2 or 3, but you need to define what. you can't leave it open.
