The term 'FISH ORGAN' refers to two organs found in fish: the GILL and the FIN. The GILL is a respiratory organ that allows fish to extract oxygen from the water. It is composed of thin filaments that are arranged in a series of plates, and it is typically four letters long. The FIN is an organ used for locomotion and balance. It is composed of thin, flexible membranes supported by bony or horny spines, and it is typically three letters long.