campus

Campus, campi, m., second declension. Flat open ground: farmland when a farmer says it, and in military prose the level space where an army can deploy and fight, which is why campus and acies keep company.

It is worth holding apart from ager, which is land as property, soil and boundaries. Campus is land as a surface you can see across. In Rome the Campus Martius outside the old walls was the drill ground, the voting ground and the place for public exercise, and it is that sense of an enclosed institutional field that eventually gave English campus. French champ, champagne and campaign come from the same word, a campaign being a war fought out in the open.