Overnight means during the night, or something that happens very quickly, as if in a single night.

When you stay overnight at a friend's house, you sleep there and return home the next day. An overnight camping trip means you'll be gone for one night. Hospitals sometimes keep patients overnight for observation, monitoring them through the night hours.

The word also describes sudden, dramatic change. When a song becomes an overnight sensation, it seems to explode in popularity instantly, though usually the success took longer than one night to build. A business might become an overnight success, meaning it grew famous remarkably fast. Athletes sometimes say they became good overnight, though they actually practiced for years before anyone noticed.

People use overnight to emphasize speed and surprise. “Rome wasn't built overnight” means important things take time. If someone expects results overnight, they may be unrealistic about how long real progress takes.