- 1: The act or process of excreting.
2a: Something eliminated by the process of excretion that is composed chiefly of urine or sweat in mammals including humans and of comparable materials in other animals, characteristically includes products of protein degradation (as urea or uric acid), usually differs from ordinary bodily secretions by lacking any further utility to the organism that produces it, and is distinguished from waste materials (as feces) that have merely passed into or through the alimentary canal without being incorporated into the body proper.
b: A waste product (as urine, feces, or vomit) eliminated from an animal body: excrement--not used technically.
Definition from: Merriam-Webster's Medical Dictionary by Merriam-Webster Inc.
See also Understanding Medical Terminology.