A runtime error occurs when a program is running, causing it to stop unexpectedly. This type of error happens due to various reasons, such as trying to divide by zero, accessing a file that doesn't exist, or using incorrect data types. Unlike syntax errors, which are detected before the program runs, runtime errors only appear during execution. When a runtime error occurs, the program may crash or display an error message. Developers often use debugging tools to identify and fix these errors. Understanding runtime errors is essential for improving the reliability of software applications and ensuring a smooth user experience.