circiter

Circiter is an adverb of approximation used with numbers, and also a preposition taking the accusative for a rough time or place: circiter meridiem, circiter Kalendas. As an adverb it does not touch the case of what it qualifies, so circiter mille milites is still nominative and circiter decem dies still accusative - the word simply floats alongside.

Caesar uses it constantly, which is why it is commoner in military narrative than anywhere else. The distinction worth keeping is against circa: in Cicero, circa is spatial, "around a thing", and circiter is numerical, "roughly so many". Livy already blurs it, and by the time scholarship writes circa 1200 the classical division has gone entirely. Fere and ferme overlap too, but they hedge a statement rather than a count.