The Collaborative International Dictionary of English v.0.48:

Reasoning \Rea"son*ing\, n.
   1. The act or process of adducing a reason or reasons; manner
      of presenting one's reasons.
      [1913 Webster]

   2. That which is offered in argument; proofs or reasons when
      arranged and developed; course of argument.
      [1913 Webster]

            His reasoning was sufficiently profound. --Macaulay.
      [1913 Webster]

   Syn: Argumentation; argument.

   Usage: Reasoning, Argumentation. Few words are more
          interchanged than these; and yet, technically, there
          is a difference between them. Reasoning is the broader
          term, including both deduction and induction.
          Argumentation denotes simply the former, and descends
          from the whole to some included part; while reasoning
          embraces also the latter, and ascends from the parts
          to a whole. See Induction. Reasoning is occupied
          with ideas and their relations; argumentation has to
          do with the forms of logic. A thesis is set down: you
          attack, I defend it; you insist, I reply; you deny, I
          prove; you distinguish, I destroy your distinctions;
          my replies balance or overturn your objections. Such
          is argumentation. It supposes that there are two
          sides, and that both agree to the same rules.
          Reasoning, on the other hand, is often a natural
          process, by which we form, from the general analogy of
          nature, or special presumptions in the case,
          conclusions which have greater or less degrees of
          force, and which may be strengthened or weakened by
          subsequent experience.
          [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Reason \Rea"son\ (r[=e]"z'n), v. i. [imp. & p. p. Reasoned
   (r[=e]"z'nd); p. pr. & vb. n. Reasoning.] [Cf. F.
   raisonner. See Reason, n.]
   1. To exercise the rational faculty; to deduce inferences
      from premises; to perform the process of deduction or of
      induction; to ratiocinate; to reach conclusions by a
      systematic comparison of facts.
      [1913 Webster]

   2. Hence: To carry on a process of deduction or of induction,
      in order to convince or to confute; to formulate and set
      forth propositions and the inferences from them; to argue.
      [1913 Webster]

            Stand still, that I may reason with you, before the
            Lord, of all the righteous acts of the Lord. --1
                                                  Sam. xii. 7.
      [1913 Webster]

   3. To converse; to compare opinions. --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

reasoning
    adj 1: endowed with the capacity to reason [syn: intelligent,
           reasoning(a), thinking(a)]
    n 1: thinking that is coherent and logical [syn: reasoning,
         logical thinking, abstract thought]

Moby Thesaurus II by Grady Ward, 1.0:

85 Moby Thesaurus words for "reasoning":
   Vernunft, abstract thought, act of thought, analysis, analytic,
   arguments, brain, brains, brainwork, cerebral, cerebration,
   cogitation, conceit, conception, conceptive, conceptual,
   conceptualization, creative thought, discourse of reason,
   discursive reason, endopsychic, esprit, excogitation, explanation,
   explication, gray matter, head, headpiece, headwork,
   heavy thinking, hypothesis, idea, ideation, imageless thought,
   intellect, intellection, intellectual, intellectual exercise,
   intellectual faculty, intellectualization, intelligence,
   intelligent, internal, logic, mens, mental, mental act,
   mental capacity, mental labor, mental process, mentality,
   mentation, mind, noesis, noetic, noological, nous, phrenic,
   postulate, power of reason, premises, psyche, psychic, psychologic,
   ratio, ratiocination, ratiocinative, ratiocinatory, rational,
   rationale, rationality, rationalization, reason, reasoning faculty,
   reasons, smarts, spiritual, straight thinking, subjective, theory,
   thinking, thinking aloud, thinking out, thought, understanding