A "click handler" is a piece of code that responds to mouse click events on a webpage or application. When a user clicks on an element, such as a button or a link, the click handler executes a specific function or action. This allows developers to create interactive features, like opening a new page or displaying a message. In web development, click handlers are often implemented using programming languages like JavaScript. They can be attached to various elements in the Document Object Model (DOM), enabling dynamic user experiences. By managing user interactions effectively, click handlers enhance the overall functionality of websites and applications.