1. An extremely fast-moving or fast-flowing part of a river, caused by a steep descent in a riverbed: For their summer holiday, Carl and Hans planned to kayak on the river and to shoot the rapids through the treacherously fast flowing water.
2. The part of a river where the current moves faster than the common current: The rapids in the river were filled with large boulders and Eric and Stuart decided to portage around them by carrying their boat and supplies overland.

A rapids is a section of a river where it loses elevation or height over a relatively short distance and the stream goes down steeply causing an increase in water flow and turbulence.