Imperial Cathedral in its original context means a cathedral linked to the german emperors.
The Romanesque cathedrals on the Rhine in Mainz, Worms and Speyer are called Imperial Cathedrals.
Mainz Cathedral was built by archbishop Willigis of Mainz, then regent of the empire, in the years after 975. Henry IV also contributed to the building after 1081. Mainz Cathedral was coronation site for Philipp von Schwaben, Frederick II, and Henry Raspe. Also Henry II (in 1002), and Conrad II (in 1024) were crowned in the cathedral at Mainz, but both presumably in the preceding building, since the present cathedral was consecrated in 1038 after their coronations.
Speyer Cathedral was built starting in 1030 by Conrad II as a family vault for the Salian dynasty and was later extended by Henry IV.
Worms Cathedral houses the tombs of emperor Conrad's II family and also got its status because of its size and glory.
Less well known is that the Salian built basilica in Koenigslutter, under the reign of Lothair III, also is traditionally called Imperial Cathedral. The Cathedral in Bamberg, built under reign of Henry II, where the only canonized married couple has their grave, also has the same status. Aachen Cathedral and the Frankfurt Cathedral(Cathedral of Saint Bartholomew) in Frankfurt were also described as imperial cathedrals because they were the coronation cathedrals of emperors and kings.