This is another term attached to a German vessel by the British. The German term for these ships was Panzerschiffe armored ships. They were built more or less following the restrictions imposed on Germany in the Treaty of Versailles.

The German theory for these ships was simple. Pack enough punch to take out the enemy. Be fast enough to catch him if needed. But also be fast enough to run away if needed. All of this had to fit within the restrictions of the treaty. So, the German heavy cruiser was born.

They carried the armor and armament of a battle ship. They looked like a battleship. However they were not as heavy as a battleship. The British saw them for what they were scaled down battleships. So, they began calling them pocket battleships because they could fit in your pocket. That is actually an understatement, but these new ships were the forerunner of a new style of ship.

Compare the Alaska class cruiser to the German pocket battleships and there are many similarities.