What term describes the rapidity of motion or action?

The term that best describes the rapidity of motion or action is "celerity." This word specifically refers to swiftness, speed, or promptness in movement. It emphasizes the quickness with which something is done or the rate at which motion occurs. The root of the word relates to the idea of being quick, and it is often used in more formal or literary contexts.

Velocity, while also related to motion, refers specifically to the speed of something in a given direction, which introduces a vector element that "celerity" does not. Acceleration is concerned with the rate of change of velocity, essentially dealing with how quickly an object can increase its speed, rather than the overall swiftness of motion itself. Agility refers more to the ability to move quickly and easily, often implying dexterity or nimbleness rather than simply the speed of motion. Thus, celerity is the most appropriate choice for describing rapidity in a more general sense.