An assertion is a statement or declaration that expresses a belief or opinion. In programming, assertions are used to verify that certain conditions hold true during the execution of a program. If an assertion fails, it typically indicates a bug or an unexpected situation that needs to be addressed. Assertions help developers catch errors early in the development process by checking assumptions made in the code. They are often used in testing frameworks and can improve code reliability by ensuring that specific conditions are met before proceeding with further operations.