A content gap is a topic or keyword competitors rank for that your site doesn't cover at all, or covers weaker than what's currently ranking.
Content gap analysis is comparative by nature. It requires looking at what's actually ranking for a target keyword and identifying what those pages do that yours doesn't (or doesn't yet exist to do). The most valuable gaps are usually where the current top-ranking content is outdated, thin, or missing a format (a tool, a calculator, an original dataset) that would genuinely serve the query better, not just where a keyword has zero results at all.
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