The Collaborative International Dictionary of English v.0.48:

Placidity \Pla*cid"i*ty\, n. [L. placiditas: cf. F.
   placidit['e].]
   The quality or state of being placid; calmness; serenity.
   --Hawthorne.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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