A user agent in SEO refers to the software or program that acts as a representative for a user when accessing a website. It is responsible for sending information about the user’s device, browser, and operating system to the website’s server. This information is then used by search engines to determine the relevancy and compatibility of the website’s content with the user’s device. In other words, the user agent plays a crucial role in optimizing a website for different devices and improving its search engine ranking.