The following definitions are take from Merriam-Webster's Third New International Dictionary.
Rationality
1 : the quality or state of being rational "man's rationality is not a higher faculty T imposed upon his animal natureó Grace De Laguna"
2 : acceptability to reason : REASONABLENESS "gives a rationality T and a justification to the universeó J.W.Krutch" "the social rationality that T alone can save usó A.L.Locke"
3 : something (as an opinion, belief, or practice) that is rational ó usually used in pl "a race T between the rationalities of technology and the irrationalities of styleó Lewis Mumford
Rational
1 : having reason or understanding : REASONING "rational creature" "a rational being" "embryo had a vegetable life, then an animal life, and finally a rational lifeó S.F.Mason"
2 a : of, relating to, or based upon reason "provide a literary as well as a rational educationó G.K.Chalmers" "rational analysis of the problemó R.C.Doty"
b : using medical treatments based on reason or general principles ó used especially of an ancient school of physicians; opposed to empirical
3 : involving only multiplication, division, addition, and subtraction and only a finite number of times : not involving a surd or indicated but not extractable root "3 and 2 + 3/5 are rational expressions"
4 a : agreeable to reason : INTELLIGENT, SENSIBLE "gives a quite rational explanation of the passageó Modern Language Notes" "a rational T world trade policyó Nation's Business" "the advantages of a rational orthographyó C.H.Grandgent" b : RATIONALISTIC
5 : capable of being measured in terms of the mora in Greek and Latin prosody : having the normal ratio between arsis and thesis
Reason
1 a : an expression or statement offered as an explanation of a belief or assertion or as a justification of an act or procedure "gave reasons that were quite satisfactory"
b : a consideration, motive, or judgment inducing or confirming a belief, influencing the will, or leading to an action or course of action : a rational ground or motive "will mention a reason for this situation" "the reason that this is so should now be clear" "a good reason to act as you do" "does not know the reason why"
c : a sufficient ground of explanation or of logical defense; especially : a general principle, law, or warranted presumption that supports a conclusion, explains a fact, or validates a course of conduct "brilliantly outlined the reasons that supported his client's action"
d : the thing that makes some fact intelligible : CAUSE "the reason for the tides lies in the gravitational pull of the moon and the sun" e : a sane or sound view or consideration "that's a reason that you should keep in mind"
2 a (1) : the power of comprehending, inferring, or thinking especially in orderly, sensible, rational ways "was afraid that his reason might be deranged" "must use reason to solve this problem" : the ability to trace out the implications of a combination of facts or suppositions "a reason that is far beyond her years"(2) : proper exercise of the intellective faculty in accordance with right judgment : right use of the mind : right thinking "attempted to bring her to reason"(3) : a sane or sound mind marked by the right use of the intellective faculty : reasonableness and sanity of the mind "was afraid she would lose her reason"
b (1) : a distinct cognitive faculty coordinate with perception and understanding : human intelligence or intellect(2) : the sum of the intellectual powers(3) : universal or general rationality of all minds viewed as a whole(4) : mind or intelligence viewed as a rational soul pervading the whole of nature or of the universe
c : NOUS, NOESIS "a transcendent ideal that only reason beyond experience can conceiveó John Dewey"
d Aristotelianism (1) : the function of the soul that is pure actuality, operates on the material furnished by passive reason, is immaterial and imperishable, and enjoys impersonal immortality ó called also active reason, creative reason(2) : the function of the soul which operates with sensuous images and in which concepts are merely potential so that they need to be formed by active reason ó called also passive reason(3) : PRACTICAL REASON
e Scholasticism (1) : INTELLECT(2) : RATIO
f Kantianism & German idealism : the highest faculty of the mind especially when conceived of as the faculty of framing general conceptions or of directly apprehending universals ó distinguished from understanding
3 logic : PREMISE; especially : MINOR PREMISE