- allergyA disorder of the immune system causing adverse reactions to substances (allergens) not harmful to most and marked by the body's production of histamines and associated with atopy, anaphylaxis, and asthma; any condition of hypersensitivity to a substance.
- allosaurusA large, carnivorous dinosaur, of genus Allosaurus, that lived in the Jurassic period.
- allochthonousOriginating in a place other than where it is found.
- allochlorophyllFormer name of chlorophyll b
- allocrineRelating to or denoting foreign hormones being taken up and eliciting a response in an organism (for example, hormones in breast milk.)
- allodoxaphobiaFear of other people's opinions.
- allopatryThe condition of being allopatric.
- allophylicRelating to a race or a language neither Aryan nor Semitic.
- alloscutumthe flexible part of the dorsal body wall behind the rigid scutum in a hard tick, particularly a female — the alloscutum is the part of the body wall that must stretch to accommodate the blood that the tick swallows when it engorges in preparation for laying eggs.
- allostasisThe process of achieving stability, or homeostasis, in the body, through physiological or behavioral change.
- allotropeAny form of an element that has a distinctly different molecular structure to another form of the same element, with different physical properties and often different chemical properties.