The crossword solution for the term GILL OR LUNG is ORGAN, which is a five letter word. An ORGAN is a part of an organism that performs a specific function. In the case of GILL OR LUNG, the ORGAN in question is a respiratory ORGAN. Gills are organs found in aquatic animals that allow them to extract oxygen from the water, while lungs are organs found in terrestrial animals that allow them to extract oxygen from the air.