This is another bastard of the age of steam. It is the direct invention of John Fisher, First Lord of the Admiralty at the turn of the 20th century. The battlecruiser was a ship with battleship heavy guns, and cruiser swift speed. The trade off being to get the speed and keep the heavy guns your had to sacrifice armor. The theory being that speed was armor. The theory was sadly wrong as was proven in the HMS Hood disaster.
From the Epiphany Trek point of view the term is an inexact one. Any large ship whose primary purpose is to fight can be called a battle cruiser. The term is most often used to describe Klingon ships. They like it.
A battlecruiser was a type of Federation Starship in the 23rd CenturyBased on the Hull of the Constitution class starship, the Battlecruiser was focused on combat, discarding many of the multi-role abilities of the hull. these modifications made Federation battlecruisers fierce opponents, but not as versatile in the field.
In combat these ships were wildly successful, but when responding to other types of emergencies, these ships suffered from their focus on battle.
Retired with the Khitomer Accords, the type most often aged out in the reserve, although many were aggressively scavenged to keep more multi-role Constitution , Constellation , Miranda and Challenger class cruisers in operation.The survivors of these types, except for a few museum displays are now being scraped out to provide raw materials for newer ships.
The type is replaced by a Heavy Frigate.