proelium

Proelium, proelii, n., second declension. This corpus spells it praelium about as often as proelium, and it is one word either way: medieval scribes had stopped hearing any difference between ae and oe, so the two diphthongs swap freely on the page.

The set idiom is proelium committere, "to join battle", where committere means bringing two sides together rather than committing anything. Latin grades the scale of fighting and expects you to respect it: pugna is a fight, proelium a single engagement, bellum the entire war, so bellum gerere and proelium committere are not swappable. Nothing survives into the modern languages; battle, bataille and batalla all descend from late Latin battualia, "fencing practice".