Definition
rel='nofollow' is an HTML attribute on <a> tags that hints crawlers not to use the link for ranking/authority transfer. Originally introduced to fight comment-spam SEO. Google now treats it as a hint, not a directive. AI engines mostly ignore nofollow when building their citation graph: they still crawl and read nofollowed pages.
<a href='https://sponsor.com' rel='nofollow sponsored'>Our partner</a>: the link works but doesn't transfer SEO authority.
How to optimize
Use rel='sponsored' for paid links (replaces nofollow for ads). Use rel='ugc' for user-generated content. Use nofollow only when you want to disclaim editorial endorsement.