TRANQUIL
placid, quiet, still, tranquil, smooth, unruffled
(adjective) (of a body of water) free from disturbance by heavy waves; “a ribbon of sand between the angry sea and the placid bay”; “the quiet waters of a lagoon”; “a lake of tranquil blue water reflecting a tranquil blue sky”; “a smooth channel crossing”; “scarcely a ripple on the still water”; “unruffled water”
calm, unagitated, serene, tranquil
(adjective) not agitated; without losing self-possession; “spoke in a calm voice”; “remained calm throughout the uproar”; “he remained serene in the midst of turbulence”; “a serene expression on her face”; “she became more tranquil”; “tranquil life in the country”
Source: WordNet® 3.1
Etymology
Adjective
tranquil (comparative tranquiler, superlative tranquilest)
Free from emotional or mental disturbance.
Calm; without motion or sound.
Synonyms
• (free from emotional disturbance): calm, peaceful, serene, steady
• (calm; without motion or sound): peaceful
Antonyms
• (free from emotional disturbance): agitated
Source: Wiktionary
Tran"quil, a. Etym: [L. tranquillus; probably fr. trans across, over
+ a word akin to quietus quiet: cf. F. tranquille. See Quiet.]
Definition: 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.
Source: Webster’s Unabridged Dictionary 1913 Edition