Extra-Vehicular Activity is any activity done by an astronaut or cosmonaut outside a spacecraft beyond the Earth's appreciable atmosphere.
In the Martian they have special spacesuits that they wear when exploring the area beyond The Hab called EVA suits. Chris Beck is the EVA specialist and has done 5 spacewalks/EVAs at the SpaceX Station. Not to be confused with an EMU suit (Extravehicular Mobility Unit).
