1560s, "an onion," from Middle French bulbe (15c.), from Latin bulbus "bulb, bulbous root, onion," from Greek bolbos "plant with round swelling on underground stem." Extended 1660s to "spherical underground part of an onion, lily, etc." Expanded by 1800 to "swelling in a glass tube" (thermometer bulb, light bulb, etc.).
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