A pulse oximeter is a medical device

Medical device

A medical device is a product which is used for medical purposes in patients, in diagnosis, therapy or surgery . Whereas medicinal products achieve their principal action by pharmacological, metabolic or immunological means. Medical devices act by other means like physical, mechanical, thermal,...

 that indirectly monitors the oxygen saturation of a patient's blood

Blood

Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....

 (as opposed to measuring oxygen saturation directly through a blood sample) and changes in blood volume in the skin, producing a photoplethysmograph

Photoplethysmograph

A photoplethysmogram is an optically obtained plethysmogram, a volumetric measurement of an organ. A PPG is often obtained by using a pulse oximeter which illuminates the skin and measures changes in light absorption...

. It is often attached to a medical monitor

Medical monitor

A medical monitor or physiological monitor or display, is an electronic medical device used in medical monitoring that displays the monitored data, and may or may not have the ability to transmit the data on a monitoring network...

 so staff can see a patient's oxygenation at all times. Most monitors also display the heart rate. Portable, battery-operated pulse oximeters are also available for home blood-oxygen monitoring.