The Collaborative International Dictionary of English v.0.48:

Sagacity \Sa*gac"i*ty\, n. [L. sagacitas. See Sagacious.]
   The quality of being sagacious; quickness or acuteness of
   sense perceptions; keenness of discernment or penetration
   with soundness of judgment; shrewdness.
   [1913 Webster]

         Some [brutes] show that nice sagacity of smell.
                                                  --Cowper.
   [1913 Webster]

         Natural sagacity improved by generous education. --V.
                                                  Knox.
   [1913 Webster]

   Syn: Penetration; shrewdness; judiciousness.

   Usage: Sagacity, Penetration. Penetration enables us to
          enter into the depths of an abstruse subject, to
          detect motives, plans, etc. Sagacity adds to
          penetration a keen, practical judgment, which enables
          one to guard against the designs of others, and to
          turn everything to the best possible advantage.
          [1913 Webster]

WordNet (r) 3.0 (2006):

sagacity
    n 1: the mental ability to understand and discriminate between
         relations [syn: sagacity, sagaciousness, judgment,
         judgement, discernment]
    2: the trait of forming opinions by distinguishing and
       evaluating [syn: judiciousness, sagacity,
       sagaciousness]