The "touchstart" event is a type of user interaction in web development that occurs when a user places a finger on a touch-sensitive screen, such as those found on smartphones and tablets. This event is part of the Touch Events API, which allows developers to create responsive and interactive applications for touch devices. When the "touchstart" event is triggered, it can be used to initiate actions like opening menus, starting animations, or registering a touch gesture. This event is essential for enhancing user experience on mobile devices, as it enables more intuitive interactions compared to traditional mouse events.