rapid

As an adjective, it emphasizes the rate of change or speed of movement. It is more clinical and objective than "fast," often appearing in professional, scientific, or economic contexts (e.g., "rapid inflation" or "rapid eye movement"). While "quick" often refers to a single brief action, "rapid" typically describes a sustained process that occurs over a short duration. As a noun, it is almost always used in the plural ("rapids") and evokes an image of chaos, danger, and physical turbulence. It describes a specific geographical feature where gravity and narrow channels force water into a violent, frothing state.