https://moz.com/learn/seo/on-site-seo

SEO is the practice of optimizing elements on a website (as opposed to links elsewhere on the Internet and other external signals collectively known as "off-site SEO") in order to rank higher and earn more relevant traffic from search engines. On-site SEO refers to optimizing both the content and HTML source code of a page.

https://moz.com/learn/seo/on-site-seo