Other forms: imperials
Imperial carries the implication of royalty, usually pertaining to an empire; the emperor or empress is referred to as "your imperial highness."
The word imperial has a lofty feel, and though it's often used of royalty, it is sometimes used to refer to anything extremely large or impressive. It was once the name of a large suitcase that was too big to fit inside a coach and so was tied on top. Think of something imperial as being something so upper-class that it's almost to the point of royalty — and sometimes almost to the point of being a joke.