A group of related content pieces that together establish topical authority.
A content cluster is a pillar page (broad overview, targeting a big keyword) plus many related sub-pages (specific long-tail keywords within the topic) all interlinked. This pattern tells Google: 'we know this topic deeply, not just one page of it.' Clusters rank better than isolated articles, especially on competitive topics.
Writing 30 pages on one topic works better than writing 30 pages on 30 random topics. Clusters compound. Pick a topic your business cares about, build a cluster over 6-12 months, and you own the category in search.