Adjective tranquil has 2 senses
- calm, serene, tranquil - characterized by absence of emotional agitation; "calm acceptance of the inevitable"; "remained serene in the midst of turbulence"; "a serene expression on her face"; "she became more tranquil"; "tranquil life in the country"
Antonym: unpeaceful (indirect, via peaceful)
- placid, quiet, still, tranquil, unruffled - free from disturbance; "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"
Antonym: stormy (indirect, via calm)
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