A trench is a long, narrow ditch dug into the ground. When workers install underground pipes or cables, they dig trenches to hold them. Archaeologists dig trenches at dig sites to carefully uncover artifacts layer by layer. Farmers sometimes dig trenches for irrigation, creating channels that carry water to their crops.

The word became especially important during World War I, when opposing armies dug elaborate systems of trenches for protection. Soldiers lived in these muddy ditches for months, sheltered from enemy fire. Trench warfare describes this grim style of fighting, where armies faced each other across a devastated no-man's-land between their trenches. The horrible conditions in WWI trenches led to diseases like trench foot, caused by standing in cold, wet conditions for days.

Deep ocean trenches are similar formations underwater. The Mariana Trench in the Pacific Ocean plunges nearly seven miles down, the deepest point on Earth. These ocean trenches form where one tectonic plate slides beneath another.

The word can also be a verb: workers might trench a field to prepare it for planting, or trench through soil to lay a foundation.