Blood oxygen level is a term referring to the concentration of oxygen

Oxygen

Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 in the 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....

. The human body requires and regulates a very precise and specific balance of oxygen in the blood.

This balance is maintained for the most part by chemical processes in the body to sustain aerobic metabolism and life.

Pulse oximetry

Pulse oximetry

Pulse oximetry is a non-invasive method allowing the monitoring of the oxygenation of a patient's hemoglobin.A sensor is placed on a thin part of the patient's body, usually a fingertip or earlobe, or in the case of an infant, across a foot....

is a method used to measure the concentration of oxygen in the blood.