tranquil C1

/ˈtɹæŋ.kwɪl/ Calm, quiet, and peaceful.

tranquillize C2+

/ˈtræŋkwəlaɪz/ to make calm or sedate, especially with a drug

trank C2+

/træŋk/ slang for a tranquilizer or sedative drug

tranquillity C2+

/træŋˈkwɪləti/ the state of being calm, peaceful, and untroubled

tranquilness C2+

/ˈtræŋkwɪlnəs/ the quality of being calm and free from disturbance