Let a be a point in the domain of a function. The function is said to be continuous at if .
Otherwise f is said to be discontinuous at . The function may be discontinuous at a for various reasons:
i) may not exist, may not exist, or both may not exist.
ii) both may exist, but may not be equal.
iii) both may exist, and be equal, but not be equal to f(a).
If f is continuous at all points of its domain it is simply said to be continuous. (Sometimes for emphasis mathematicians say f is continuous everywhere).
Note that it is a silly question to ask whether or not a function is continuous at a point which is not in the domain. For example is continuous at x=1?