Source : Webster's Revised Unabridged Dictionary (1913)

Placidity \Pla*cid"i*ty\, n. [L. placiditas: cf. F.
   placidit['e].]
   The quality or state of being placid; calmness; serenity.
   --Hawthorne.

Source : WordNet®

placidity
     n 1: a feeling of calmness; a quiet and undisturbed feeling [syn:
           {placidness}]
     2: a disposition free from stress or emotion [syn: {repose}, {quiet},
         {serenity}, {tranquillity}, {tranquility}]