An outsider is a non-elemental creature that comes from another plane of existence, primarily the outer planes. They are supernatural creatures that tend to have a strong association with their alignment (e.g. slaadi are incarnations of chaos). Many good-aligned outsiders are called celestial, while evil outsiders are referred to as fiendish or demonic, a vestige of their original inspirations (angels, devils, and demons).
The most prevalent influence of outsiders on the game is through clerics, as by tradition a deity is a powerful outsider (hence it is an outsider that grants spells to a cleric). Outsiders can also be encountered directly, often the result of a summoning (such as the player spells gate, greater planar binding, and lesser planar binding). It is also possible for some outsiders to travel to the material plane by their own will, or to be sent by other outsiders.
Due to their supernatural nature, outsiders often have spell resistance, damage reduction, and some form of (elemental) damage resistance. They are subject to turning, though, by those who possess the planar turning feat or the good or evil domains. They are also subject to banishment, which returns an outsider to its home plane. (Theoretically, this spell should not function against an outsider already on its home plane, but the standard spell does not account for that.)
See also: Category:Outsiders