Formal:
* Justification: This emphasizes a logical reason or explanation.
* Apology: This expresses regret for wrongdoing.
* Rationalization: This suggests a less-than-completely honest reason.
* Explanation: This focuses on providing details about the situation.
* Defense: This implies a reason for protecting oneself from blame.
Informal:
* Reason: A simple and straightforward way to say why something happened.
* Pretext: This suggests a false or misleading reason.
* Story: This is a casual way to refer to a reason, often implying it might not be entirely true.
* Out: A slang term for an excuse, often used for avoiding something.
* Cop-out: A negative term for an excuse that avoids responsibility.
Other possibilities:
* Justification
* Rationale
* Explanation
* Reasoning
* Defense
* Pretext
* Motive
* Claim
* Justification
The best word to use depends on the specific situation and the tone you want to convey.