What is a U boat?

U boat is used for German submarines in both World Wars. It is a translation of German U-Boot which is short for Unterseeboot.

A U-Boat is the English expression for the German U-Boot (these languages are related historically - this explains the "boat" and "boot" - meaning the same). The "U" is short for "Untersee-Boot", "unter" equals "sub", "see" equals "mare"... seen? A U-Boot is German for submarine.