The Collaborative International Dictionary of English v.0.48:

Ratiocination \Ra`ti*oc`i*na"tion\
   (r[a^]sh`[i^]*[o^]s"[i^]*n[=a]"sh[u^]n), n. [L. ratiocinatio:
   cf. F. ratiocination.]
   The process of reasoning, or deducing conclusions from
   premises; deductive reasoning.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ratiocination
    n 1: the proposition arrived at by logical reasoning (such as
         the proposition that must follow from the major and minor
         premises of a syllogism) [syn: conclusion,
         ratiocination]
    2: logical and methodical reasoning

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "ratiocination":
   abstract thought, act of thought, brainwork, cerebration,
   cogitation, conceit, conception, conceptualization, conclusion,
   creative thought, deduction, deductive reasoning, demonstration,
   discourse, discourse of reason, discursive reason, excogitation,
   headwork, heavy thinking, idea, ideation, illation,
   imageless thought, induction, inductive reasoning, intellection,
   intellectual exercise, intellectualization, judgment,
   logical thought, mental act, mental labor, mental process,
   mentation, noesis, philosophy, proof, rationalism, rationality,
   rationalization, rationalizing, reason, reasonableness, reasoning,
   sequitur, sophistry, specious reasoning, straight thinking,
   sweet reason, thinking, thinking aloud, thinking out, thought