.According to the SRD, an outsider is at least partially composed of the essence (but not necessarily the material) of some Plane other than the Material+Plane. Some creatures start out as some other type and become outsiders when they attain a higher (or lower) state of spiritual existence.
In Sigilian Cant, an Outsider is anyone from the Prime Material Plane who doesn't yet belong to a Faction. Once you join a faction, you're not an Outsider anymore; you're a Factioneer, and planars can start respecting you. There have been attempts to organize Outsiders into a faction of their own, but they've always failed. It seems no one wants to advertise being Clueless.
Sources:
Planescape Campaign Setting
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