How a treaty might distinguish battlecruisers

How could a naval treaty define battlecruisers separately from battleships?

It would probably need a speed limit for battleships or a measure of armour density, because gun calibre does not reliably distinguish the types.

The two types often carry the same guns, so calibre cannot provide a clean legal boundary. Battlecruisers generally accept less armour coverage in exchange for speed, but that design relationship is difficult to express in a treaty. A legal definition could impose a battleship speed ceiling or measure hull and armour density. Either method would create verification problems and give designers incentives to exploit the chosen formula.