The "Touchstart" event is a type of user interaction in web development that occurs when a user touches the screen of a device, such as a smartphone or tablet. This event is part of the Touch Events API, which allows developers to create responsive applications that react to touch gestures. It is often used to initiate actions like opening menus or triggering animations. When a touch is detected, the "Touchstart" event is fired before any movement occurs, providing a way to capture the initial touch point. This event can be handled using JavaScript, enabling developers to enhance user experience by making applications more interactive and engaging on mobile devices.