\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.
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).