Outer Senshi is a fan canon term typically referring to Sailor Uranus, Sailor Neptune, Sailor Pluto, and Sailor Saturn. It is possibly an abbreviation of the official term "Senshi of the Outer Solar System," which refers to the same group of Senshi. However, unlike that term which strictly refers to those four characters, fans have been known to include Sailor Chibi Moon, Sailor Chibi Chibi Moon, and even Sailor Jupiter in lists of the "Outer Senshi."
The lack of awareness that this is a fan canon and not an official term sometimes leads to arguments over which characters are "officially" Outer or Inner Senshi.
The Outer Senshi also guarded the three Talismans, the key to the appearance of Sailor Saturn, the bringer of death. When the three Talismans were brought together, Saturn would bring down her glaive. In Sailor Moon S, the mission of the Outer Senshi was to prevent these Talismans from being reunited, so that Sailor Saturn could not bring about the Silence.