- a depression of significant depth in the ocean floor, usually about 2.0–2.5 miles (3–4 kilometers) deeper than the surface of the surrounding seafloor and about 30–60 miles (50–100 kilometers) wide, with a length that may span thousands of miles or kilometers: an integral element of plate tectonics.
- a long narrow steep-sided depression in the earth's oceanic crust, usually lying above a subduction zone
