1) a cause, explanation, or justification.
2) good or obvious cause to do something.
3) the power of the mind to think, understand, and form judgements logically.
4) (one's reason) one's sanity.
5) what is right, practical, or possible.
6) think, understand, and form judgements logically.
7) (reason out) find a solution (to a problem) by considering possible options.
8) (reason with) persuade with rational argument.