The Collaborative International Dictionary of English v.0.48:

Sanity \San"i*ty\, n. [L. sanitas, from sanus sound, healthy.
   See Sane.]
   The condition or quality of being sane; soundness of health
   of body or mind, especially of the mind; saneness.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sanity
    n 1: normal or sound powers of mind [syn: sanity, saneness]
         [ant: insanity]

Moby Thesaurus II by Grady Ward, 1.0:

63 Moby Thesaurus words for "sanity":
   IQ, apprehension, balance, caliber, capacity, common sense,
   comprehension, conception, cool head, coolheadedness, coolness,
   deductive power, due sense of, esemplastic power, good sense,
   horse sense, ideation, integrative power, intellect,
   intellectual grasp, intellectual power, intellectualism,
   intellectuality, intelligence, intelligence quotient, knowledge,
   level head, levelheadedness, lucidity, marbles, mental age,
   mental capacity, mental grasp, mental health, mental ratio,
   mentality, mind, mother wit, native wit, normality, plain sense,
   power of mind, practical mind, practical wisdom, practicality,
   rationality, reason, reasonableness, reasoning power, saneness,
   scope of mind, sense, senses, sensibleness, sober-mindedness,
   soberness, sobriety, sound mind, soundness, stability,
   thinking power, understanding, wit



Bouvier's Law Dictionary, Revised 6th Ed (1856):

SANITY, med. jur. The state of a person who has a sound understanding; the
reverse of insanity.
     2. The sanity of an individual is always presumed. 5 John. R. 144; 1
Pet. R. 163; 1 Hen. & M. 476; 4. Cowen, R. 207; 4 W. C. C. R. 262. See 9
Conn. 102; 9 Mass. 225; 3 Mass. 336 1 Mass. 71; 8 Mass. 371; 8 Greenl. 42;
15 John. 503; 4 Pick. 32.