Sure it is, if it affects all users at once, and makes your system unusable. If I dial a phone number and get a busy signal, that's an "error" of sorts. If ever

Sure it is, if it affects all users at once, and makes your system unusable. If I dial a phone number and get a busy signal, that's an "error" of sorts. If every Verizon user gets that busy tone on every call, that's an outage.