Free from commotion. Of water: Having an unruffled surface, without waves or violent current; motionless or flowing imperceptibly.

Of water, air, etc.: Without motion or current; still, standing. (See also dead water, n.)

Of weather: Calm, serene. Of water: Smooth, still. Obsolete.

Of water, the sea, etc.: Not broken or turbulent; free from big waves or roughness; running or flowing evenly, calmly, or gently.

Of water, the sky, etc.: not disturbed or agitated; not set in motion or churned up; remaining in a calm or tranquil state. Also in figurative…

Esp. of a body of water: not rippled, smooth.

Of water: having no current.

Hydrodynamics and Aerodynamics. Used predicatively as adj. in preceding senses.