1. Anatomy. Either one of a pair of organs in the dorsal region of the vertebrate abdominal cavity, functioning to maintain proper water and electrolyte balance, regulate acid-base concentration, and filter the blood of metabolic wastes, which are then excreted as urine.
2. The kidney of certain animals, eaten as food.
3. An excretory organ of certain invertebrates.
4. Temperment; kind: a person of the same kidney.

[Middle English kidenei.]

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.