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. Today, you may have even own a trench coat, a kind of raincoat first worn by soldiers.