A trench is a deep and narrow hole, or ditch, in the ground, like the kind soldiers on frontlines might dig to give themselves shelter from the enemy. A natural trench may also be a deep hole on the bottom of the ocean. The verb trench means to dig or to cut into, but you will most often hear the word as a noun, particularly relating to soldiers. The phrase trench warfare became popular around World War I, when technological advances in weapons changed the way that wars were fought.