Technically, the Latin word imperator was the title of every Roman general.
Imperator is not synonymous of emperor, although is the root of it. Through the history of Rome, the title evolved to the meaning of a general especially able who had won an enormous victory. Traditionally, it was the troops in the field that proclaimed a man imperator, an acclamation necessary for a general to apply the senate for a triumph.
Since a triumph is the ambition of every Roman politician, history is full of cases were legions are bribed to call someone imperator.
See also: imperium