Rapid means happening with great speed or in a brief period of time. Think of Rapid Eye Movement (REM sleep), when your eyes are darting back and forth quickly — trying to follow your fast-paced dreams.
Rapid comes from the Latin word rapere, which means "to hurry away" or "to seize." If you are first to raise your hand in class with the correct answer, your teacher will be pleased with your rapid response. Your pulse may be rapid after you run a quick sprint, and after a rapid run-up in the stock market you may decide to sell some shares and lock in the profit.