Manual action
A manual action is a penalty applied by a human reviewer at Google, reported in Search Console, that can demote or deindex pages or a whole site.
A manual action is issued when a Google reviewer (not an algorithm) confirms a page or site violates the spam policies. For link building the relevant types are "unnatural links to your site" and "unnatural links from your site." Unlike algorithmic suppression, a manual action appears explicitly in the Security & Manual Actions report in Google Search Console, so you always know when you have one.
It matters because the impact can be severe, ranging from ranking demotions on affected pages to full deindexing of the site. The presence of a notice also means there is a defined path back: fix the underlying problem, then submit a reconsideration request explaining what you changed. Algorithmic issues have no such request process, which is why a manual action, while alarming, is at least actionable.
A nuance worth knowing: recovery is rarely instant. For an unnatural-links action you typically need to remove or disavow the offending links and write an honest reconsideration request describing the cleanup. Reviewers can reject vague or half-hearted requests, so document the specific links, the outreach you attempted, and the link scheme you dismantled. Address the root cause, not just the symptom.