Exit rate is actually measured by Google Analytics. In SEO, exit rate is defined as the percentage of people who leave your website from a specific page, that specific page can be home page or any other web page of a website.
Exit rate is the percentage of visitors who leaves your website from specific page.
The main difference between exit rate and bounce rate is bounce rate is the percentage of visitors who only visit one page and leaves from that page only While in exit rate there may be possible that visitors visit more than one page.