Outsider is the generic term for a supernatural being whose heritage can be traced to one or more of the upper or lower planes -- that is, the celestials (or aedra) and the fiends (or daedra).
The defining trait of all outsiders is that they are "immortal" -- that is, they will not age, grow sick or die of natural causes. People who carry some outsider blood, but do not share these characteristics, are called Plane-touched.
For more information:
Races of MK2K