principium

Principium, principii, n., second declension, from princeps, which is primus plus capio, "the one who takes first place". So a principium is where a thing takes its start.

It leads two lives. The plain temporal one - in principio, the phrase that opens both Genesis and John in the Vulgate, and a principio, "from the outset". And the philosophical one, where the plural principia renders the Greek archai, the first elements or fundamental laws, which is the sense in Newton's title. Military prose adds a third: principia is the front line of a legion, and then the headquarters at the centre of a camp. English took principle and principal from the same stem and has been confusing them ever since; Latin is not responsible.