Dictionary.com's Word of the Year is...
late 14c., "exercise of the power of reason; act or process of thinking logically;" also "an instance of this;" verbal noun from reason (v.).
c.1200, "intellectual faculty that adopts actions to ends," also "statement in an argument, statement of explanation or justification," from Anglo-French resoun, Old French raison "course; matter; subject; language, speech; thought, opinion," from Latin rationem (nominative ratio) "reckoning, understanding, motive, cause," from ratus, past participle of reri "to reckon, think," from PIE root *re(i)- "to reason, count" (cf. Old English rædan "to advise; see read (v.)).
Meaning "sanity; degree of intelligence that distinguishes men from brutes" is recorded from late 13c. Sense of "grounds for action, motive, cause of an event" is from c.1300. Middle English sense of "meaning, signification" (early 14c.) is in the phrase rhyme or reason. Phrase it stands to reason is from 1630s. Age of Reason "the Enlightenment" is first recorded 1794, as the title of Tom Paine's book.
early 14c., resunmen, "to question (someone)," also "to challenge," from Old French raisoner "speak, discuss; argue; address; speak to," from Late Latin rationare "to discourse," from ratio (see reason (n.)). Intransitive sense of "to think in a logical manner" is from 1590s; transitive sense of "employ reasoning (with someone)" is from 1847. Related: Reasoned; reasoning.
in philosophy, the faculty or process of drawing logical inferences. The term "reason" is also used in several other, narrower senses. Reason is in opposition to sensation, perception, feeling, desire, as the faculty (the existence of which is denied by empiricists) by which fundamental truths are intuitively apprehended. These fundamental truths are the causes or "reasons" of all derivative facts. According to the German philosopher Immanuel Kant, reason is the power of synthesizing into unity, by means of comprehensive principles, the concepts that are provided by the intellect. That reason which gives a priori principles Kant calls "pure reason," as distinguished from the "practical reason," which is specially concerned with the performance of actions. In formal logic the drawing of inferences (frequently called "ratiocination," from Latin ratiocinari, "to use the reasoning faculty") is classified from Aristotle on as deductive (from generals to particulars) and inductive (from particulars to generals).