When you search Google for a keyword, the results include paid ads on the page and other listings that Google considers to be the best answers to what you're looking for. If a domain is in that list of unpaid search results, we consider the search term to be one of the domain's organic keywords. 

Specifically, we include the keyword if the domain ranked within the top 50 results.
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The chart of organic keywords on the SEO Overview helps you see how the number changed over time and against Google algorithm updates.