imperator

Imperator, imperatoris is third declension masculine, an agent noun off impero, "I command", with the same -tor suffix as victor, auctor and ductor.

Its history is the trap. Under the Republic it was a battlefield acclamation, not a rank: troops hailed a victorious general imperator on the field, and he kept the title until he had celebrated his triumph, then dropped it. Augustus took it into his name permanently, and from there it slid into meaning the emperor himself - French empereur, Spanish emperador, English emperor. So in Caesar or Livy, imperator means "general", and rendering it "emperor" imports the wrong constitution into the sentence. Note too that the Greek East did not translate it as imperator but used autokrator, which is a different claim altogether.