An outpost was typically any frontier base or settlement that was usually minimally equipped, and which served many purposes, including research and defense. On many worlds, an outpost was the precursor to a colony.
Calder II was the location in 2370 of an outpost that consisted of a small science station and had limited defensive capabilities. Because Calder II was a Federation outpost, it was defended by Starfleet. The outpost had a type-four deflector shield, which protected it and the surrounding ruins, as well as two phaser banks. (TNG: "Gambit, Part I")
The Son'a had an outpost on Devos II in 2375 that was the site of a ketracel-white production facility. In that year, Weyoun had Damar send Dominion ships to the outpost to protect it against a possible Federation attack. (DS9: "Penumbra")