Abducens nerve
Sixth pair of cranial nerve of vertebrates. Almost entirely motor, supplying external rectus muscles of the eye ball.

Abductor
Any muscle that moves a limb away from the body. An example of an abductor is the abductor pollex, which moves the thumb outward.

Abiogenesis
(Spontaneous generation) The origin of living from non-living matter as biopoiesis.

Abiotic factors
Characterized by the absence of life. includes temperature, humidity, pH, and other physical and chemical influences.

Abjection
The separation of a spore from its stalk, forcibly by the fungus.

Aboral
Opposite the mouth.

Abortion
The spontaneous or induced expulsion of a foctus before it becomes viable outside the uterus or womb.

Abscess
A collection of pus surrounded by an inflamed area in any tissue or organ of an animal.

Absolute refractory period
The brief period during the discharge or a nerve impulse when the neuron cannot fire again.

Absorption
The movement of fluid or a dissolved substance across a cell membrane. In animals, for example, soluble food materials is absorbed into the circulatory system through cells lining the alimentary canal. In plants, water and mineral salts are absorbed from the soil by the roots. See osmosis.