late 14c., "having a commanding quality," from Old French imperial, emperial "imperial; princely, splendid; strong, powerful" (12c.), from Latin imperialis "of the empire or emperor," from imperium "empire" (see empire).
Meaning "pertaining to an empire" (especially Rome's) is from late 14c.; by 1774 of Britain's. Meaning "of imposing size or excellence" is from 1731. Imperial presidency in a U.S. context traces to Arthur Schlesinger Jr.'s book on the Nixon administration (1974). Related: Imperially. The noun is from 1520s as "member of the emperor's party;" 1670s as the name of gold coins issued by various imperial authorities.