A falls is a waterfall, a natural place where water cascades from a height. Niagara Falls is one of the world's most well-known falls.
The noun falls can be singular or plural: "We saw four small falls flowing down the side of the mountain." Falls are formed when a river or stream flows over a cliff or a high, rocky area. Some falls occur when snow melts and water cascades down the face of a mountain. The word falls comes from the water falling, from an Old English root word: feallan, "to drop from a height."