princeps

Princeps, principis, m., third declension, from primus plus capio: the one who takes first place. It works as an adjective, "first, chief", as readily as a noun, and the stem is princip-, not princeps-.

The word carries the whole awkward history of the Roman constitution. Under the Republic the princeps senatus was simply the senator whose name headed the roll. Augustus adopted princeps as his everyday title precisely because it was not rex and not dominus, and the regime he built is called the Principate for that reason. Medieval Latin shifts it again to a prince or territorial ruler. English took prince and principal from it; principle looks like a cousin but comes from principium instead.