MSH: An element with atomic symbol O, atomic number 8, and atomic weight [15.99903; 15.99977]. It is the most abundant element on earth and essential for respiration.,CSP: use this term only for elemental or gaseous oxygen (02); for other compounds see OXYGEN COMPOUND.,NCI: A colorless, odorless gas. It is needed for animal and plant life. Oxygen that is breathed in enters the blood from the lungs and travels to the tissues.,NCI: An element with atomic symbol O, atomic number 8, and atomic weight 16.