Webster's Revised Unabridged Dictionary · 1913

Tranquil

Tranquil , adjective

[Latin tranquillus; probably from trans across, over + a word akin to quietus quiet: compare French tranquille. See Quiet.]

Quiet; calm; undisturbed; peaceful; not agitated; as, the atmosphere is tranquil; the condition of the country is tranquil.
A style clear, tranquil, easy to follow. — De Quincey