Similar words
velocidad
rapidez is more about quickness or swiftness in general; velocidad is more technical or measurable, like speed in km/h, physics — both mean "speed."
Similar in other languages
Portuguese
velocidade
velocidade is the Portuguese word for the same thing — both mean "speed."
Practice
1 What does rapidez mean?
speed
2 Fill the gap: Lo hizo con ___.
Full sentence: Lo hizo con rapidez. (He did it quickly.)
3 Which fingerprint chart matches rapidez?
rapidez: neutral register, common frequency, general domain, neutral connotation.