Final answer:
A trench is a deep depression on the sea floor, formed when one tectonic plate is forced underneath another. The Mariana Trench is an example of a trench.
Step-by-step explanation:
A trench is a topographic depression of the sea floor that is relatively narrow in width but very long, usually associated with convergent plate boundaries. Trenches are formed when one tectonic plate is forced underneath another plate in a process called subduction. This causes the seafloor to buckle and form a deep trench. An example of a trench is the Mariana Trench in the western Pacific Ocean, which is the deepest part of the world's oceans.