An anaerobic organism or anaerobe is any organism

Organism

In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...

 that does not require 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...

 for growth. It could possibly react negatively and may even die if oxygen is present. There are three types:

• obligate anaerobe
  Obligate anaerobe
  Obligate anaerobes are microorganisms that live and grow in the absence of molecular oxygen; some of these are killed by oxygen. -Metabolism:...
  
  s, which cannot use oxygen for growth and are even harmed by it
• aerotolerant organisms, which cannot use oxygen for growth, but tolerate the presence of it
• facultative anaerobe
  Facultative anaerobic organism
  A facultative anaerobic organism is an organism, usually a bacterium, that makes ATP by aerobic respiration if oxygen is present but is also capable of switching to fermentation...
  
  s, which can grow without oxygen but can utilize oxygen if it is present

Obligate anaerobes may use fermentation

Fermentation (biochemistry)

Fermentation is the process of extracting energy from the oxidation of organic compounds, such as carbohydrates, using an endogenous electron acceptor, which is usually an organic compound. In contrast, respiration is where electrons are donated to an exogenous electron acceptor, such as oxygen,...

 or anaerobic respiration

Anaerobic respiration

Anaerobic respiration is a form of respiration using electron acceptors other than oxygen. Although oxygen is not used as the final electron acceptor, the process still uses a respiratory electron transport chain; it is respiration without oxygen...

.
Aerotolerant organisms are strictly fermentative.
In the presence of oxygen, facultative anaerobes use aerobic respiration; without oxygen some of them ferment, some use anaerobic respiration.
There are many anaerobic fermentative reactions.

Fermentative anaerobic organisms mostly use the lactic acid fermentation pathway:

C₆H₁₂O₆

Glucose

Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...

 + 2 ADP

Adenosine diphosphate

Adenosine diphosphate, abbreviated ADP, is a nucleoside diphosphate. It is an ester of pyrophosphoric acid with the nucleoside adenosine. ADP consists of the pyrophosphate group, the pentose sugar ribose, and the nucleobase adenine....

 + 2 phosphate → 2 lactic acid

Lactic acid

Lactic acid, also known as milk acid, is a chemical compound that plays a role in various biochemical processes and was first isolated in 1780 by the Swedish chemist Carl Wilhelm Scheele. Lactic acid is a carboxylic acid with the chemical formula C3H6O3...

 + 2 ATP

Adenosine triphosphate

Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...

The energy released in this equation is approximately 150 kJ per mol, which is conserved in regenerating two ATP from ADP per glucose.