From The Collaborative International Dictionary of English v.0.48 [gcide]:
Tranquil \Tran"quil\, a. [L. tranquillus; probably fr. trans
across, over + a word akin to quietus quiet: cf. F.
tranquille. See Quiet.]
Quiet; calm; undisturbed; peaceful; not agitated; as, the
atmosphere is tranquil; the condition of the country is
tranquil.
[1913 Webster]
A style clear, tranquil, easy to follow. --De Quincey.
[1913 Webster] Tranquilization
From WordNet (r) 3.0 (2006) [wn]:
tranquil
adj 1: (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" [syn: placid, quiet,
still, tranquil, smooth, unruffled]
2: 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" [syn: calm, unagitated,
serene, tranquil]
From The Free On-line Dictionary of Computing (8 July 2008) [foldoc]:
TRANQUIL
1966. ALGOL-like language with sets and other extensions, for
the Illiac IV. "TRANQUIL: A Language for an Array Processing
Computer", N.E. Abel et al, Proc SJCC 34 (1969).