A term for a full body, air tight suit used by aliens or humans entering into a social encounter in atmospheres or environments that are toxic or unbreathable by the wearer. The term was coined, or at least first widely used, in Babylon 5 to describe the outfit worn by the Vorlon ambassador Kosh. As no social functions take place outside of Earth-normal conditions this is purely a matter of Science Fiction.
An encounter suit is similar to a hazmat suit or a spacesuit but it is not generally meant to work in situations quite as extreme as those. They can be armored but their purpose is to allow a being access to areas that would contain untenable atomspheres for them. Many alien species in B5 must wear similar garb in order to interact with fellow diplomats. Encounter suits are meant to be as much an adornment to the wearer as the clothing worn by their peers is to them; it is the body that is put forth to the community of aliens with which you are interacting. Because of this an encounter suit can be as baroque as the wearer sees fit as its function is simply to provide a livable environment for them.
The Vorlon encounter suit was a mask that the Vorlons put on to hide their true form from the species that they controlled/looked after. It is wide and nearly cylindrical, rising in the back in two large humps. At the center is a depression, from which rises the helmet which looks somewhat pyramidal, pointing in the direction of vision. The center of the face is an "eye" which allowed the wearer to signal emotions, states of mind etc. On the chest of the suit was presumably the translation device Kosh used to communicate with those around him.
The idea of an encounter suit goes beyond Babylon 5; it is unreasonable to have diplomats, in tense social situations in spacesuits that are intended for use in more extreme conditions while being clumsier and less apt to social graces. The encounter suit was not meant to be armor, against attackers or the ravages of vacuum, its purpose was for meetings between species with incompatible biochemistries. As such it deserves separate consideration from functional personal life-support.