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]

Moby Thesaurus II by Grady Ward, 1.0:

67 Moby Thesaurus words for "sagacity":
   acuity, acumen, acuteness, anticipation, apperception, astuteness,
   background, blaseness, cogency, comprehension, contemplation,
   discernment, discretion, envisagement, envisionment, experience,
   farseeingness, farsightedness, forecast, foreglance, foregleam,
   foreglimpse, forehandedness, foreseeing, foresight,
   foresightedness, grasp, incisiveness, insight, judiciousness,
   longheadedness, longsightedness, looking ahead, past experience,
   penetration, perception, perceptiveness, percipience,
   perspicaciousness, perspicacity, perspicuity, perspicuousness,
   practical knowledge, practice, prediction, preparation,
   prepublication, preview, prevision, prospect, prospection,
   providence, provision, prudence, readiness, sagaciousness,
   sageness, sapience, seasoning, sensibility, sensitivity,
   sophistication, tempering, trenchancy, understanding, wisdom,
   worldly wisdom