1. Music.
   1. 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.
   2. Any one of various other instruments, such as the electronic organ, that resemble a pipe organ either in mechanism or sound.
2. Biology. A differentiated part of an organism, such as an eye, wing, or leaf, that performs a specific function.
3. An instrument or agency dedicated to the performance of specified functions: The FBI is an organ of the Justice Department.
4. An instrument or a means of communication, especially a periodical issued by a political party, business firm, or other group.

[Middle English, from Old French organe and from Old English organe, both from Latin organum, tool, instrument, from Greek organon.]

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.