1. A condition characterized by abnormally low levels of oxygen in the blood and tissues: One form of hypoxia is altitude sickness in which the deficiency of oxygen in the blood and tissues is caused by being in very high altitudes for too long.
2. In ecology, a condition in which inadequate environmental oxygen is available to an organism.

The "Dead Zone" off the Louisiana coast mapped during the last week of July, 2006, is reported to be 6,662 square miles; or about the size of Connecticut and Rhode Island combined, Dr. Nancy Rabalais, Chief Scientist for Northern Gulf of Mexico Hypoxia Studies reported on Friday. The low oxygen waters extended from near the Mississippi River to the Louisiana-Texas border.

Agricultural runoff in the vast area drained by the Mississippi River contributes most of the nitrogen, phosphorus and other nutrients that feed a population boom of algae. As the algae die, they fall to the bottom. Their decay consumes oxygen faster than currents can bring it down from the surface.