Neil Tennant coined this phrase, I believe, to refer to that period in an act's career when they can do no wrong: hit after hit, riding the zeitgeist like Roy Rogers on Trigger, where an artists just keeps coming with the hit singles non stop.

For ex. in the UK PSB had 3 number 1's from 87/88 even a christmas number 1 which is a big deal there, and everything else went top 3 till Domino Dancing went number 7 and after that he feels the singles had no gurantee to get top 3.

So let's play the "when was their Imperial Phase?" game

To Start This i'll Offer

U2: Unforgettable Fire to Zooropa

Led Zeppelin: IV- In Through The Out Door

Prince- 1999- Around The World In A Day

Stevie Wonder Signed, Sealed, Delivered To Songs In The Key Of Life

Van Halen- 1984 to Balance

R. Kelly- 12 Play to Chocolate Factory

Phil Collins- No Jacket Requied To Both Sides

Michael Jackson- Off The Wall to History

Bruce Springsteen- Born In The U.S.A To Human Touch

Lionel Richie- Self Titled To Dancing On The Ceiling

Boys II Men- Cooleyhighharmony To Evolution

The Who- Who's Next To The KId's Are Alright

George Michael- Make It Big To Listen Without Prejudice

R.E.M- Green To Monster

Hall & Oates- Voices To Change Of Season

Whitney Houston- Whitney Houston To My Love Is Your Love

Mariah Carrey- Self titled to Glitter

Cyndi Lauper- She's So Unusual To A Night To Remember

Ozzy Osbourne- Bark At The Moon To Osmosis