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A long-tail keyword is a keyword phrase that aims to capture search traffic from a specific, often 3+ word search query. 

Long-tail keywords are used to target niche demographics rather than mass audiences. The "long-tail" refers to the search demand curve  -- as keyword terms become more specific and detailed, search volume becomes less competitive, but searcher intent becomes higher.

Because of that, they provide both short-term and long-term benefits.

Since they're very specific, they allow you to rank highly in search results for specific topics relatively quickly while still gaining ground on head terms. Plus, they tend to attract highly qualified traffic that's more likely to convert to leads and customers.