Oxygen saturation or dissolved oxygen (DO) is a relative measure of the amount of oxygen (as O) that is dissolved in a given medium. It can be measured with a dissolved oxygen such as an oxygen sensor. Or it can be measured with an optode in liquid media, usually water. The standard unit is milligrams per litre (mg/l) or parts per million (ppm). Fish, invertebrates, plants, and aerobic bacteria all require oxygen (dissolved in water) for respiration. In medicine, oxygen saturation refers to "oxygenation". This means when oxygen molecules () enter the tissues of the body.