ordo

Ordo, ordinis is a third declension noun and it is MASCULINE, which is worth flagging because most Latin nouns ending in -o with a genitive in -inis or -ionis are feminine. The genitive ordinis gives you the stem ordin- and every other form: ordinem, ordine, plural ordines, ordinum.

Its senses run from the concrete to the abstract without changing shape. It is a row of things, a rank of soldiers in the line, the sequence in which events happen, and then a social class: ordo senatorius and ordo equester are the senatorial and equestrian orders, and the phrase is standard in political prose. The ablative ordine, "in order, in sequence", and extra ordinem, "irregularly, out of turn", are worth learning whole.

A frequency warning. Ordo sits this high partly because our corpus mixes classical prose with administrative and ecclesiastical Latin, where it is the word for a religious order, for holy orders, and for the fixed sequence of a rite (ordo missae). Caesar and Cicero use it, but not at this rate. It gives English order, ordain, ordinary, coordinate and subordinate.