Anaerobic is defined as being able to exist without, or not requiring air or oxygen. Most often, anaerobic is used to define a cell or organism that lives without oxygen. The term may also be used to describe a process or function that occurs without air.
The word anaerobic is from the French word, anaérobie, coined by Louis Pasteur in 1863. It is from the Greek an- ("without") + aer ("air") + bios ("life"). The opposite word is "aerobic," meaning requiring oxygen or air.
In humans, anaerobic organisms are typically found in the gastrointestinal tract. Some anaerobic bacteria produce important toxins.