- An element with atomic symbol O, atomic number 8, and atomic weight 16. It is the most abundant element on earth and essential for respiration.
Definition from: MeSH via Unified Medical Language System at the National Library of Medicine
- A colorless tasteless odorless gaseous element that constitutes 21 percent of the atmosphere and is found in water, in most rocks and minerals, and in numerous organic compounds, that is capable of combining with all elements except the inert gases, that is active in physiological processes, and that is involved especially in combustion processes -- symbol O.
Definition from: Merriam-Webster's Medical Dictionary by Merriam-Webster Inc.
Related discussion in the Handbook
See also Understanding Medical Terminology.