noun

1. an explanation or justification of an act, idea, etc.
2. a cause or motive
3. the ability to think, form judgments, draw conclusions, etc.
4. sound thought or judgment; good sense
5. normal mental powers; a sound mind; sanity

Etymology: ME reisun < OFr < L ratio, a reckoning, reason: see read

intransitive verb

1. to think coherently and logically; draw inferences or conclusions from facts known or assumed
2. to argue or talk in a logical way

transitive verb

1. to think logically about; think out systematically; analyze
2. to argue, conclude, or infer: now usually with a clause introduced by that as the object
3. to support, justify, etc. with reasons
4. to persuade or bring by reasoning (into or out of)