arvum

Arvum, arvi is second declension neuter, originally the neuter of the adjective arvus, "ploughed", from aro, "to plough". So an arvum is not any old field: it is land under the plough, as against a pascuum for grazing or a campus for open flat ground. Ager is the broad legal and geographical word, arvum the agricultural one.

It is overwhelmingly a poetic word, and overwhelmingly plural: arva, arvis. Virgil's Georgics run on it, and by extension it stretches to mean any wide expanse, even the sea or the fields of the dead. English arable comes from the same verb, through arabilis, and that is the sense to keep in mind whenever you meet arva in verse.