Mortals are individuals of normal human birth who are subject to age and death. He or she is born, lives and dies as opposed to an Immortal, who has unending life. Mortals can occasionally become immortal. Callisto became immortal upon eating the Golden Apples, while Psyche became immortal after drinking Ambrosia. It is also possible for immortals to become mortal, although this is much rarer – it's actually more common for immortals to die than to lose their immortality. Cheiron the Satyr, Ares, Aphrodite and even Zeus were immortals who became mortal for varying reason.
In spite of their limitations, mortals can acquire impressive skills or even power in their own right. Xena is perhaps the single most skilled and dangerous mortal ever to challenge the gods, while individuals such as Lao Ma or Merlin have achieved power while still ostensibly remaining mortal.