Music An instrument consisting of a number of pipes that sound tones when supplied with air and a keyboard that operates a mechanism controlling the flow of air to the pipes. Also called pipe organ. Music Any one of various other instruments, such as the electronic organ, that resemble a pipe organ either in mechanism or sound. Biology A differentiated part of an organism, such as an eye, wing, or leaf, that performs a specific function. An instrument or agency dedicated to the performance of specified functions: The FBI is an organ of the Justice Department. An instrument or a means of communication, especially a periodical issued by a political party, business firm, or other group.