Going outside the spacecraft while in space — and also the period of time you spend out there. So you can count them ('her third spacewalk') and you can measure them ('a six-hour spacewalk'). Nobody actually walks: the astronaut floats, holds on to rails, and is tied to the craft by a cable so as not to drift away. The 'walk' is the everyday word people reached for because 'going outside' is what walking means to us on the ground. Engineers have their own term, EVA, short for extravehicular activity, and you will hear that in mission recordings; 'spacewalk' is the word for the rest of us, used in the news and in ordinary talk. It works as a verb too — astronauts spacewalk, they went spacewalking — though the noun with 'do', 'go on', or 'carry out' is far more common.