When you do something rapidly, you do it very quickly. A rapidly moving train is going at full speed.

You may notice that the grass in your yard grows rapidly during the rainy summer days and more slowly when there's a heat wave. You might also get a speeding ticket for driving too rapidly in a school zone. In either case, something's happening fast. The adverb rapidly comes from the adjective rapid, "moving quickly," with the Latin root rapidus, "hasty, swift, fierce, or impetuous."

Definitions of rapidly
  1. adverb
    with rapid movements
    synonyms: apace, chop-chop, quickly, speedily
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    easy, slow, slowly, tardily
    without speed (`slow' is sometimes used informally for `slowly')
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