Imperator
The title of imperator was originally the rough equivalent of commander under the Roman Republic. Later, it became a part of the titulature of the Roman emperors as their praenomen. The Roman emperors generally based their authority on multiple titles and positions, rather than preferring any single title. Nevertheless, imperator was used relatively consistently as an element of a Roman ruler's title throughout the Principate and the later Roman Empire. It was abbreviated to "IMP" in inscriptions. The word is an agentive form of the verb, meaning "to order, to command". The English word emperor derives from imperator via French.
File:Weltliche Schatzkammer Wien.JPG|thumb|The Imperial Crown of the Holy Roman Empire with the inscription "ROMANORU IMPERATOR AUG" on the right side of the arch.
File:Wien Hofburg brama.jpg|thumb|Golden dedication inscription at the Äusseres Burgtor of the Hofburg Palace in Vienna of "FRANCISCUS. I. IMPERATOR. AUSTRIAE. MDCCCXXIV.", who ruled as Francis II, last Holy Roman Emperor until 1806
''Imperatores'' in the ancient Roman Kingdom
When Rome was ruled by kings,to be able to rule, the king had to be invested with the full regal authority and power. So, after the comitia curiata that was held to elect the king, the king also had to be conferred the imperium.
''Imperatores'' in the Roman Republic
In Roman Republican literature and epigraphy, an imperator was a magistrate with imperium. During the later Roman Republic and during the late Republican civil wars, imperator mainly was the honorific title assumed by certain military commanders. After an especially great victory, an army's troops in the field would proclaim their commander imperator, an acclamation necessary for a general to apply to the Senate for a triumph. After being acclaimed imperator, the victorious general had a right to use the title after his name until the time of his triumph, where he would relinquish the title as well as his imperium.Since a triumph was the goal of many politically ambitious commanders, republican history is full of cases where legions were bribed to call their commander imperator. The title of imperator was given firstly to Aemilius Paulus in 189 BC, for his campaigns against the Lusitanians from 191–189 BC. Next, it was given to Lucius Cornelius Sulla in 95 BC as governor or pro consul of the Cilician province for his returning of Ariobarzanes I to the Cappadocia throne. Then in 90 BC to Lucius Julius Caesar, in 84 BC to Gnaeus Pompeius Magnus, in 60 BC to Gaius Julius Caesar, relative of the previously mentioned Lucius Julius Caesar, during the 50s BC to Gaius Julius Caesar, in 45 BC again to Gaius Julius Caesar, in 43 BC to Decimus Junius Brutus, and in 41 BC to Lucius Antonius. In AD 15 Germanicus was also imperator during the empire of his adoptive father Tiberius.
Use in eastern Rome and other post-Roman states
After the Roman empire collapsed in the West in the fifth century, Latin continued to be used as the official language of the Eastern Roman Empire. The Roman emperors of this period were referred to as imperatores in Latin texts, while the word basileus and autokrator were used in Greek.Beginning in 1077, Alfonso instituted the use of the style ego Adefonsus imperator totius Hispaniae and its use soon became regular. This title was used throughout the period 1079–81, which represents the peak of his imperial pretensions before his capture of the city of Toledo, ancient capital of the Visigoths. In 1080, he introduced the form ego Adefonsus Hispaniarum imperator, which he used again in 1090. His most elaborate imperial title was ego Adefonsus imperator totius Castelle et Toleto necnon et Nazare seu Alave.
After the Ottoman Empire conquered both the Balkan peninsula and Constantinople, the Turkish ruler claimed to have become the Caesar of Rome. In the fifteenth century Bayezid II established diplomatic relations with some Christian European states, and sent a document to the King of Poland in which he used the titles Sultan Dei gratia Asie, Grecie etc. Imperator Maximus. Like his predecessor, Selim I titled himself imperator in diplomatic correspondence due to his military successes.