A Reason

1. A cause:
   1. That which causes something: an efficient cause, a proximate cause.
   2. A motive for an action or a determination.
   3. An excuse: a thought or a consideration offered in support of a determination or an opinion; that which is offered or accepted as an explanation.
   4. (logic) A premise placed after its conclusion.
2. (uncountable) Rational thinking (or the capacity for it); the cognitive faculties, collectively, of conception, judgment, deduction and intuition. https://en.wikipedia.org/wiki/Reason
3. (obsolete) Something reasonable, in accordance with thought; justice.
4. (mathematics, obsolete) Ratio; proportion.

Synonyms

• (that which causes): cause
• (motive for an action): rationale, motive
• (thought offered in support): excuse