The common use of mortal is as the antithesis to immortal. An immortal is someone who will, at least, never die of natural causes. At other times immortality includes invincibility to weapons and/or diseases. Mortals are normal humans, who are in mortal peril of all these things; age, disease and physical trauma.
In the World of Darkness, however, the designation is commonly attributed to non-supernaturals, even when some supernatural groups are not immortal as such. The question is usually raised when discussing the imbued or mages, since they are closest to normal humans in a physical sense.
Other terms for mortals, used by the various kinds of supernatural beings, include humans, kine, the People of the Map, sleepers, the masses and the quick.