Where a battlecruiser departs from a cruiser

Is the distinction between a cruiser and a battlecruiser basically the caliber of the main weapons?

Caliber is part of it, but battlecruisers are also much larger and better protected than cruisers while remaining short of battleship standards.

Main-armament caliber is an important dividing line, though it is not the only one. A battlecruiser should be substantially larger than an ordinary cruiser, with better subdivision and armor, while remaining below a battleship's overall fighting standard. The Soviet Kirov and Pyotr Velikiy type offers an example of ships commonly called battlecruisers because they tower over contemporary cruisers. The answer cautions against classifying warships from one weapon statistic alone.