The term FISH'S BREATHING ORGAN is a four letter word, GILL. Gills are organs used by fish to breathe underwater. They are thin, feathery structures that absorb oxygen from the water and expel carbon dioxide. Gills are located on either side of the fish's head and are made up of many small filaments. The gills allow the fish to extract oxygen from the water and exchange it with carbon dioxide in their bloodstream.