Lewis & Short

tranquillĭtas, ātis, f. [tranquillus], quietness, stillness, tranquillity.

  1. I. Lit., calmness of wind or weather, a calm: tanta subito malacia ac tranquillitas exstitit, ut se ex loco movere non possent (naves), Caes. B. G. 3, 15: si proficiscatur hac tranquillitate, Cic. Ac. 2, 31, 100: maris tranquillitas intellegitur nullā ne minimā quidem aurā fluctus commovente, id. Tusc. 5, 6, 16: mira serenitas cum tranquillitate oriebatur, Liv. 26, 11, 3: summā tranquillitate consecutā, Caes. B. G. 5, 23: insidiosa, Plin. Pan. 66, 3; and in plur.: nos longis navibus tranquillitates aucupaturi eramus, Cic. Att. 6, 8, 4; cf.: securitas quae est animi tamquam tranquillitas, id. Fin. 5, 8, 23.
  2. II. Trop., calmness, quiet, serenity, tranquillity of mind or affairs (a favorite trope of Cic.): locus quietis et tranquillitatis plenissimus, Cic. de Or. 1, 1, 2: tranquillitas (animi), id est placida quietaque constantia, id. Tusc. 4, 5, 10: summa tranquillitas pacis et otii, id. Agr. 1, 8, 21; cf. Sen. Tranq. 2, 3 sq.: tranquillitas animi et securitas … tranquillitatem expetere, Cic. Off. 1, 20, 69; cf.: otium ac tranquillitatem vitae sequi, id. Mur. 27, 55: et jam ibi nequaquam eadem quies ac tranquillitas erat, Liv. 24, 27, 7: non multum ad tranquillitatem locus confert, Sen. Ep. 55, 8: illa tranquillitas vera est, in quam bona mens explicatur. id. ib. 56, 6: tranquillitatem et otium penitus auxit, Tac. Agr. 40 fin.: ad carminis tranquillitatem tamquam ad portum confugerunt, Petr. 118.
    1. B. In partic.
      1. 1. De Tranquillitate Animi, the title of a work of Seneca the philosopher.
      2. 2. Tranquillitas tua, Your Serenity, Your Serene Highness, a later title of the Roman emperors, Eutr. praef.: vestra, id. 1, 11.