#8 refers to the size of the screw or nut. A #8 is .1640" dia and comes in either 32 or 36 TPI.
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In a classroom setting it could also apply to the person sitting in seat # 8
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As Re-design said, "#8" is a standard size (in the American "Unified Screw Series", most likely UNF).
It is the size of the hole and threads. It fits an #8 bolt or machine screw.