A "div" is a fundamental element in HTML (Hypertext Markup Language) used to create sections or divisions within a webpage. It acts as a container that can hold other elements like text, images, and other HTML tags. By using divs, web developers can organize content and apply styles more effectively. Divs are often styled with CSS (Cascading Style Sheets) to control their appearance, such as size, color, and layout. This makes them essential for creating responsive designs that adapt to different screen sizes. Overall, divs help structure and enhance the visual presentation of web content.