ager

Ager, agri, m., second declension. The e of the nominative is a helper vowel that appears nowhere else: agri, agro, agrum, and the plural agri, agrorum, agris, agros. Same behaviour as liber, libri. Do not build the oblique forms off ager-.

Two scales of meaning sit in the one word. The small one is a field you plough; the large one is the territory of a state, ager Romanus, which is a political boundary rather than a farm. English took agriculture, agrarian and agronomy from it, and peregrinus, literally "one who is out per agros, across the fields", is the same root wandering off to become "pilgrim". French champ and Spanish campo come from campus instead.