Extravehicular activity, also known as EVA refers to any work done outside the hull of a space starship or habitat, and not in the breathable atmosphere of a planet. Extravehicular activities generally involve zero gravity situations, however the term is sometimes also used in regards to planetary missions in environments with no surface atmosphere, no breathable atmosphere, or other toxic conditions. (ENT: "Minefield"; Star Trek: First Contact)
Extravehicular activities usually require an environmental suit, also known as an EVA suit, though occasionally certain crewmen or members of certain species are able to engage in EVA without. For instance, Lieutenant Commander Data, an android, and The Doctor, a hologram, are able to perform in many environments where most Starfleet crewmen could not. (VOY: "Basics, Part I"; Star Trek Nemesis)