A horizontal ad unit is a type of advertising format that is commonly used on the web and in mobile apps. A horizontal ad unit is characterized by its rectangular shape and its horizontal orientation, which is typically wider than it is tall. Horizontal ad units are often used to display banner ads or display ads, and are designed to fit within the layout of a website or app, typically at the top, bottom, or sides of the page. Horizontal ad units are popular among advertisers and publishers, as they can provide a large and prominent area for displaying ads, and can be easily integrated into the design of a website or app. Horizontal ad units are also easy for users to view and interact with, as they are typically placed in areas of the page that are visible without scrolling.