Formal:
* Justification: This emphasizes a logical reason or explanation.
* Apology: This implies remorse or regret.
* Rationale: This refers to a set of reasons or logic behind an action.
* Defense: This suggests an attempt to ward off criticism or blame.
Informal:
* Reason: A simple and neutral term.
* Pretext: This implies a false or fabricated reason.
* Alibi: Used specifically to explain one's whereabouts at a specific time.
* Explanation: A broad term for any reason or clarification.
* Out: A slang term, meaning to avoid something.
Other options:
* Justification: To provide a valid reason for something.
* Explanation: To make something clear or understandable.
* Defense: To protect oneself from criticism or blame.
* Apology: To express regret or remorse.
The best synonym to use will depend on the specific situation and the tone you want to convey.