The term 'organ breathing' can be summarized by the crossword solution 'GILL, ' which is a four-letter word. Gills are organs that allow aquatic animals to breathe underwater. They are located in the head or throat of the animal and are filled with blood vessels that absorb oxygen from the water. Gills are used by fish, amphibians, and some invertebrates to extract oxygen from the water and expel carbon dioxide.