Aemilius (fem. Aemilia) was the nomen of the gens Aemilia, one of the five most important patrician families (gentes maiores) of ancient Rome.
The Aemilii were a very old clan; they were supposed to have descended from Mamercus, a son of Pythagoras who was surnamed "Aemylos" or "Aimilios" because of his refinement and speaking ability. Another version quoted by Plutarch said that Mamercus was the fourth son of Numa Pompilius, the second King of Rome, Numa being an admirer of Pythagoras and so naming a son after Pythagoras' son. The branches of the Aemilii were Barbula, Buca, Lepidus, Mamercus, Papus, Paullus, Regilus and Scaurus. Of these, the Aemilii Lepidi had become the most notable, especially at the end of the Republic.