an explanation or justification
gave a good reason for being late
a rational ground or motive
The last temptation is the greatest treason, to do the right deed for the wrong reason — T S Eliot
a fact, event, etc that brings something about; a cause
the power of comprehending, inferring, or thinking, esp in orderly rational ways; intelligence
Reason is the faculty of mind by which evidence is weighed, inferences are drawn, truth and falsity are judged, and explanations are sought and given. The exercise of reason is called rationality, and any means of arriving at judgments or beliefs which do not use reason, e.g. those purely emotionally based, are non-rational. Non-rational thought includes irrational thought, which involves judgments deliberately contrary to reason — Dr Anthony Grayling
the ability to think or judge sensibly; sanity
seems to have lost all reason
it stands to reason
it is obvious, logical, or reasonable
listen to reason
to be willing to accept advice to act sensibly
within reason
within reasonable limits
with reason
with good cause
[Middle English resoun via Old French raison from Latin ration-, ratio reason, computation, from Latin reri to calculate, determine, think]