The "keydown" event is a type of event in web development that occurs when a user presses a key on their keyboard. This event is part of the KeyboardEvent interface and is commonly used in programming to detect user input. Developers can listen for this event to trigger specific actions, such as submitting a form or navigating through a webpage. When a key is pressed down, the "keydown" event is fired before the character is added to the input field. This allows developers to implement features like keyboard shortcuts or to prevent certain keys from performing their default actions. Overall, it plays a crucial role in enhancing user interaction on websites and applications.