celeriter

Celeriter is the adverb of celer, celeris, celere, a third declension adjective of three terminations. Third declension adjectives build their adverbs with -iter, which is why the form is celeriter and not the celere a learner might expect from the neuter. Comparative celerius, superlative celerrime, with the doubled r that celer takes throughout its superlative.

Caesar leans on it heavily: "quam celerrime", as quickly as possible, is his standing instruction to himself. Distinguish it from statim and subito, which are about the moment a thing starts rather than the pace it goes at. A messenger arrives statim; an army marches celeriter.