A reason, cause, motive, or excuse for something — commonly used in everyday conversation (close to 'wake').
遅れた理由を正直に話してください。
Please honestly tell me the reason you were late.
彼が会社を辞めた理由はまだはっきりしていない。
The reason he left the company is still unclear.
そんなことをする理由がわからない。
I don't understand the reason for doing such a thing.
理由を述べるときは簡潔に話すとよい。
When stating reasons, it's good to speak concisely.