What is the term for the flat plain on the ocean floor?

The term for the flat plain on the ocean floor is indeed the abyssal plain. Abyssal plains are deep, broad, and essentially featureless areas of the ocean floor that lie at depths ranging from about 3,000 to 6,000 meters (approximately 10,000 to 20,000 feet) below sea level. They are among the flattest and smoothest regions on Earth, formed by the accumulation of sediments over millions of years, which cover the more rugged features of the seafloor.

In contrast, the continental slope refers to the steep slope between the outer edge of the continental shelf and the deep ocean floor, while the continental shelf is the gently sloping submerged perimeter of a continent, and the intertidal zone is the area that is above water at low tide and underwater at high tide. Each of these terms describes different geological features of the ocean's structure and environment, but abyssal plains specifically characterize the vast, level regions of the ocean floor.