They could have a jump discontinuity, something like that. They could just have a gap where they're not defined. So they could have a gap where they're not defined. Or maybe they actually are defined there, but it's removable discontinuity. So all of these are examples of discontinuous functions. Now if you want the more mathy understanding of that, and we've looked at this before, we say that a function, f, is continuous at some value x equals a if and only if, draw my little two-way arrows here, say if and only if the limit of f of x as x approaches a is equal to the value of the function at a. So once again, in order to be continuous there, you at least have to be defined there.