The Imperial Phase is a term often used by fans, reviewers and PSB themselves to denote a period when the group were at their commercial peak, circa 1986-8.
Neil Tennant has defined the "imperial phase" as "from It's A Sin to Heart", i.e. the period of the Actually album and its associated singles (including Always On My Mind, which was not on the album). This period included three of their four UK number one hit singles (the three songs previously mentioned) and also marks a period when they made regular entries in the US top 40.
Reviewers sometimes extend the term to include the Introspective period (during which the group maintained their run of top ten hits, as well as being very successful with productions for other artists), though Tennant does not, as he considers the Imperial Phase to have ended when Domino Dancing was not the huge hit he expected. In any case, as the term implies a period of consistent success, it does not include West End Girls, as even though it was an enormous worldwide hit, the follow-up singles were only moderately successful.