In starship classification, a heavy cruiser is a large multi-purpose starship, a term sometimes used synonymously with battle cruiser. While the Federation considered their Constitution-class starships to be heavy cruisers in their heyday, the Klingons referred to the same vessel, in their terms, as a battle cruiser. (Star Trek III: The Search for Spock)
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The term "heavy cruiser" has been used only twice throughout all of Star Trek, where it was spoken in reference to the Ambassador-class ship, and on an on-screen display of the Constitution-class. All other references to large ships have used the term "battle cruiser".
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: