The English word "reason" has two main categorical meanings. They are (1) the facts, etc, that serve as the motivation for a course of action or a statement in an argument and (2) to form judgements, conclusions or inferences; to think logically, to call into question, and to hold discourse over something. The etymology of "reason" in the first meaning is that it came into English as a loanword into Middle English as reson, resoun or reisun borrowed from Old French reisun or reson, which was derived from Latin the ration form of ratio. The meaning of "sanity" was first recorded in the late 14th century.

The etymology of "reason" in the second meaning is slightly different. It similarly entered English as a loanword into Middle English as the verb resonen borrowed from Old French raisoner, which was derived from Late Latin rationare meaning "to discourse." The meaning of "to think logically" is recorded from the 1590s. The concept "it stands to reason" is recorded from the 1630s. "The age of reason" is first recorded in Thomas Paine's book The Age of Reason in 1794. The sense of "to employ reason with someone" is recorded from 1847.

The earliest known English usage of the word reason dates to the 13th century.  In the 13th century its typical meanings were- "a statement in an argument," also "intellectual faculty that adopts actions to ends." From the late 14th century "reason" meant sanity. From the 1590s "reason" meant to think in a logical manner.

Below are examples of where the word "reason" originated.

1. From the Old French word raison
2. From the Latin word ration- meaning account, reckoning, invoice
3. From the Latin word ratus- meaning established, authoritative; fixed
4. From the Latin word reri- meaning think, regard; deem