The breathing organs that fish and other aquatic animals use to extract oxygen from water, similar to how lungs work for people.
From Old Norse 'gillar,' meaning the respiratory organs of fish. The word entered Middle English through Scandinavian trade routes and has maintained its basic meaning for over 1,000 years.
Fish gills are incredibly efficient—they extract oxygen in the opposite direction of water flow, which means fish can absorb more oxygen from water than our lungs could from air. Some fish can even breathe out of water briefly by storing water in their gill chambers!
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