Source : Webster's Revised Unabridged Dictionary (1913)

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.

         A style clear, tranquil, easy to follow. --De Quincey.

Source : WordNet®

tranquil
     adj 1: 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" [syn: {calm}, {serene}]
     2: 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" [syn: {placid}, {quiet},
         {still}, {unruffled}]

Source : Free On-Line Dictionary of Computing

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).