Gill/dʒɪl/ Definition 1 of 5Pro Tip 1/2Aquatic Breathing Remember 'gill' refers to fish breathing like humans use lungs. The shark's gills moved rapidly as it swam. Pro Tip 2/2Visible Feature 'Gill' is physical and often seen as slits on a fish's side. Each gill slit on the mackerel helps its respiratory process.