Real Dictionary

kidney
countable noun: Your kidneys are the organs in your body that take waste matter from your blood and send it out of your body as urine.
variable noun: Kidneys are the kidneys of an animal, for example a lamb, calf, or pig, that are eaten as meat.

kidney in American English
either of a pair of glandular organs in the upper abdominal cavity of vertebrates, which separate water and waste products of metabolism from the blood and excrete them as urine through the bladder
noun: either of a pair of bean-shaped organs in the back part of the abdominal cavity that form and excrete urine, regulate fluid and electrolyte balance, and act as endocrine glands

kidney in British English
noun: either of two bean-shaped organs at the back of the abdominal cavity in humans, one on each side of the spinal column. They maintain water and electrolyte balance and filter waste products from the blood, which are excreted as urine