Recedite, plebes! Gero rem imperialem! - Stand aside plebians! I am on imperial business!
Recto - On the right
Redde Caesari quae sunt Caesaris - Render unto Caesar the things that are Caesar's

Redivivus

Come back to life

Redolet lvcernam - [it] smells of the lamp - critical remark that one worked too hard on something
Reductio ad absurdum - Reduction to the absurd. (proving the truth of a proposition by proving the falsity of all its alternatives)
Referendum - Something to be referred
Regina - Queen
Regnat non regitur qui nihil nisi quod vult facit - He is a king and not a subject who does only what he wishes. (Syrus)
Regnat populus - Let the People rule