The Collaborative International Dictionary of English v.0.48:

Rationality \Ra`tion*al"i*ty\ (r[a^]sh"[u^]n*[a^]l"[i^]*t[y^];
   277), n.; pl. -ties (-t[i^]z). [F. rationalit['e], or L.
   rationalitas.]
   The quality or state of being rational; agreement with
   reason; possession of reason; due exercise of reason;
   reasonableness.
   [1913 Webster]

         When God has made rationality the common portion of
         mankind, how came it to be thy inclosure? --Gov. of
                                                  Tongue.
   [1913 Webster]

         Well-directed intentions, whose rationalities will
         never bear a rigid examination.          --Sir T.
                                                  Browne.
   [1913 Webster]

WordNet (r) 3.0 (2006):

rationality
    n 1: the state of having good sense and sound judgment; "his
         rationality may have been impaired"; "he had to rely less
         on reason than on rousing their emotions" [syn:
         rationality, reason, reasonableness]
    2: the quality of being consistent with or based on logic [syn:
       rationality, rationalness]

Moby Thesaurus II by Grady Ward, 1.0:

137 Moby Thesaurus words for "rationality":
   IQ, Vernunft, admissibility, apprehension, balance, brain, brains,
   caliber, capacity, common sense, comprehension, conception,
   contact with reality, cool head, coolheadedness, coolness,
   deduction, deductive power, deductive reasoning, demonstration,
   discourse, discourse of reason, discursive reason,
   down-to-earthness, due sense of, earthiness, esemplastic power,
   esprit, freedom from illusion, good sense, gray matter,
   hardheadedness, head, headpiece, healthy mind, horse sense,
   ideation, induction, inductive reasoning, integrative power,
   intellect, intellection, intellectual faculty, intellectual grasp,
   intellectual power, intellectualism, intellectuality, intelligence,
   intelligence quotient, justifiability, justness, knowledge,
   lack of feelings, level head, levelheadedness, logic,
   logical thought, logicality, logicalness, lucid interval, lucidity,
   matter-of-factness, mens, mental age, mental balance,
   mental capacity, mental equilibrium, mental grasp, mental health,
   mental hygiene, mental poise, mental ratio, mentality, mind,
   mother wit, native wit, normalcy, normality, normalness, nous,
   philosophy, plain sense, plausibility, positivism, power of mind,
   power of reason, practical mind, practical wisdom,
   practical-mindedness, practicality, practicalness, pragmaticism,
   pragmatism, proof, psyche, ratio, ratiocination, rationalism,
   rationalization, rationalizing, realism, reason, reasonability,
   reasonableness, reasoning, reasoning faculty, reasoning power,
   right mind, sanemindedness, saneness, sanity, scientism,
   scope of mind, secularism, sense, senses, sensibleness, smarts,
   sober senses, sober-mindedness, soberness, sobriety, sophistry,
   sound mind, sound sense, soundness, soundness of mind,
   specious reasoning, sweet reason, thinking power, understanding,
   unidealism, unromanticalness, unsentimentality, wholesomeness, wit,
   worldliness