math!!!!!!!!!

if you have anything like x=y or f(x)=y then it is all the possible values of x, you could also look at it as the input of the function, hope this is clear enough.

A good Example would be :

F(x) = 1/X

X can be any number but 0.

If X were 0 -> 1/0 -> undefined.

The domain of x is pretty much the set of numbers that will give you an answer.