Pater is an old, rather posh British way of saying “father.” It is borrowed whole from Latin pater, which already meant father. In old school stories, and in jokey talk from some older British speakers, a man might call his dad “the pater.” It is not the normal word now — father or dad is — and using it today usually sounds dated, public-school, or slightly comic. You still meet it in Latin class and in a few learned English compounds. Say it PAY-ter. English already had father from its Germanic side; pater is the same ancient root coming in again through Latin, which is why the two words look like cousins once you know to look.