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A Fermi approximation is an approximation based on loose knowledge.
A usual example is "how many piano teachers live in Los Angeles", where the method to finding the answer is to rely on an approximation of Los Angeles's population, then what percentage of the population might take piano lessons, based on an approximation of the distribution, then how many hours a week piano teachers are working.
These approximations produce an order of magnitude than might be sufficient to progress on a problem analysis at low cost.
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Range quotes Fermi approximations as something that draws from broad experience, and contributes to extending one's field of expertise.