An excuse given to justify or avoid responsibility for a mistake or fault; often has a negative connotation.
彼は遅刻した理由を言い訳したが、本当は寝坊しただけだった。
He made an excuse for being late, but in fact he had just overslept.
言い訳ばかりしていると、周りの信頼を失うよ。
If you keep making excuses, you'll lose people's trust.
そんな言い訳は聞きたくない。まず謝りなさい。
I don't want to hear that excuse. Apologize first.
An explanation or account of reasons or circumstances; not necessarily negative, can be used neutrally.
欠席の言い訳を詳しく説明してもらえますか。
Could you explain in detail the reason for your absence?
会議で問題の言い訳を求められ、彼は状況を順を追って説明した。
At the meeting he was asked for an explanation of the problem, and he described the situation step by step.
その説明は言い訳というより事情の説明に近かった。
That account was closer to an explanation of the circumstances than an excuse.