A very short time; in a day or a brief interval. Often used to say something cannot be achieved in such a short time.
語学は一朝一夕に習得できるものではない。
Languages cannot be mastered in a day.
このプロジェクトの成功は一朝一夕に成るものではない。
The success of this project won't come in a short time.
彼の技術は長年の練習の賜物であり、一朝一夕ではない。
His skill is the result of years of practice and not something achieved overnight.