A closure is a programming concept where a function retains access to its outer scope's variables even after that outer function has finished executing. This allows the inner function to remember the environment in which it was created, enabling it to use those variables later. Closures are commonly used in languages like JavaScript and Python to create private data and maintain state. Closures can be particularly useful in scenarios like event handling or asynchronous programming, where you want to preserve certain values across different function calls. They help in creating more modular and maintainable code by encapsulating functionality and reducing global variable usage.