A "div" is a fundamental building block in web development, specifically in the context of HTML (Hypertext Markup Language). It is a container element used to group other HTML elements together, allowing developers to apply styles or layout changes to multiple items at once. Divs help organize content on a webpage, making it easier to manage and design. Divs are often styled using CSS (Cascading Style Sheets), which allows for customization of their appearance, such as size, color, and positioning. By using divs, developers can create responsive layouts that adapt to different screen sizes, enhancing the user experience across devices like smartphones, tablets, and desktops.