A Trench knife is a combat knife

Combat knife

A combat knife is a fighting knife designed solely for military use and primarily intended for hand-to-hand or close combat fighting.Since the end of trench warfare, most military combat knives have been secondarily designed for utility use in addition to their original role as close-quarter combat...

 designed to kill or gravely incapacitate an enemy soldier at close quarters, as might be encountered in a trenchline or other confined area. It was developed in response to a need for a close combat weapon for soldiers conducting assaults and raids on enemy trenchlines during the First World War. An example of a World War I trench knife is the German Army's Nahkampfmesser (close combat knife).

With the outbreak of the Second World War, the trench knife, by this time usually referred to as a combat knife, proved so useful that armies continued to develop and issue new designs.