Final answer:
A steep-sided canyon in the bottom of the ocean is known as a trench, which occurs at convergent plate boundaries where one tectonic plate subducts beneath another.
Step-by-step explanation:
The steep-sided canyon in the bottom of the ocean described in the question is known as a trench. Trenches are indeed topographic depressions on the sea floor, relatively narrow but very long, and they are typically associated with convergent plate boundaries where one tectonic plate is subducting beneath another. These formations are distinct from features such as the abyssal plains, which represent the vast, flat areas of the deep ocean floor; seamounts, which are underwater mountains; and oceanic ridges, which are underwater mountain ranges created by tectonic activity at divergent plate boundaries.