The title 'Emperor' is largely a modern concept.
Julius Caesar was Dictator of the Roman Republic for two years before his assassination. His successor, Augustus, wished to maintain the façade of Republican rule, and used the title Princeps Senatus (first man of the Senate). The English word 'Emperor' derives from the Latin title Imperator, which was granted to successful generals, and (at least in the early days of the Empire) still had to be earned by the Princeps.
History does generally consider Augustus, however, to have been the first Emperor of Rome. We do commonly refer to the Roman Empire, so it's natural to refer to its head as the Emperor. Even so, to placate pedants like me, you really need to ask who "is generally considered to have been" the first Emperor (or words to that effect).
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