What the heck is a battlecruiser?

So if we're using naval parlance, a battlecruiser is a ship that has the weaponry of a battleship in a cruiser's frame, hence the name. So it has lots of punch (very close to doing blast cannon and punch-above-weight jokes, but I'm not going to do that) and the speed of a cruiser. So sort of the best of both worlds in one big package.