Heavy cruiser
From Memory Alpha, the free Star Trek reference.
In starship classification, a heavy cruiser is a large multi-purpose starship.
The Federation's Constitution-class and Ambassador-class starships were considered heavy cruisers in their heyday. (Star Trek III: The Search for Spock; TNG: "Conspiracy")
[edit] Background
The term "heavy cruiser" has been uttered only once throughout all of Star Trek. In TNG: "Conspiracy" it was spoken in reference to the Ambassador class ship.
Most other references to large ships have used the term "battle cruiser". For example, TOS: "The Trouble with Tribbles"; TNG: "The Arsenal of Freedom" (applied in the script description to the battle section of the USS Enterprise-D), "The Schizoid Man"; ENT: "Judgment"; DS9: "Soldiers of the Empire"; Star Trek VI: The Undiscovered Country, and many others.
The term heavy cruiser first came into use in the Star Fleet Technical Manual. It was used in the script description of the Klingon K't'inga-class ships in Star Trek: The Motion Picture, and was thereafter picked up by the production staff on TNG in their behind-the-scenes technical texts (such as the Deep Space Nine Technical Manual). Remnants of the original usage can be seen in background graphics on the bridge in Star Trek III.
In addition to the above, the following classes were referred to as "heavy cruisers" exclusively in the Star Trek: Deep Space Nine Technical Manual:
While the Technical Manual itself is not canon, it is a Memory Alpha permitted resource.
Rick Sternbach in an interview with Star Trek: The Magazine, stated that the Sovereign-class was designated a heavy cruiser.
