adjective /ˈtɹæŋ.kwɪl/
a) Free from emotional or mental disturbance.
Some time passed before I felt tranquil even here: I had a vague dread that wild cattle might be near, or that some sportsman or poacher might discover me.
b) Calm; without motion or sound.
that the streams which did form were clear and tranquil because fed by perennial springs from the underground supply; and that in their tranquil waters extensive peat bogs formed.
Ant: agitated
Wikipedia foundation.