Rapids are a part of a river where the water moves very fast over rocks and turns white and noisy. People say shoot the rapids or white-water rapids; boats can have a wild ride there. The word is almost always plural in ordinary English: the rapids, even when you mean one stretch. It is an outdoor word — hiking, canoeing, nature films — not a feelings word. It sits next to rapid (fast), because the water is rapid there. Though actually, you would not call a fast road 'rapids'; this picture stays with rivers, and in stories sometimes other rushing water. It is not rapidity (the speed itself) and not a rapid test (that rapid only means 'quick').