The battlecruiser began as the largest and most powerful expression of the cruiser role, rather than a lightly armored battleship. Speed and firepower were supposed to let it dominate smaller cruisers, attack or defend maritime trade, scout for a battle fleet, and avoid opponents it could not defeat. As both sword and shield for a maritime economy, it could raid an enemy's commerce while protecting friendly shipping. That mission, rather than size or speed alone, provides the baseline for distinguishing battlecruisers from fast battleships.
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Battlecruisers as the supreme cruiser type
The original battlecruiser was John Fisher's supreme cruiser type, intended for worldwide trade protection, commerce raiding, naval presence, and fleet scouting.