Extended definition
Links between pages of your own site. Helps with authority distribution, crawling, user experience.
Context and application
Internal linking = the lever of total control over SEO architecture. The most powerful pages (homepage, major hubs) distribute authority to linked pages. “Orphan” pages (zero internal links) are invisible to the crawler even if they exist in the sitemap. Tactic: identify the top 10 pages by authority (DA / referring domains) and ensure they link to the pages whose ranking you want to grow. Topic hubs + supporting articles = topic clusters, the recommended structure since 2018.
In practice
any article should have 3-5 internal links to other relevant pages.