- AfricaThe continent that is south of Europe, east of the Atlantic Ocean, west of the Indian Ocean and north of Antarctica.
- agnosticDoubtful or uncertain about the existence or demonstrability of God or other deity.
- abioticNonliving, inanimate, characterised by the absence of life; of inorganic matter.
- atresiaA condition in which a body orifice or passage in the body is abnormally closed or absent.
- abiogenesisThe origination of living organisms from lifeless matter; such genesis as does not involve the action of living parents.
- aneroidNot using or containing fluid
- achalasiaSuch obstruction in the esophagus: esophageal achalasia.
- alexithymiaA deficiency in understanding, processing, or describing emotions.
- azoteNitrogen.
- abarognosisA loss of the ability to realise or estimate the weight of an object.
- abiosisThe absence of life.
- ablepharonablepharia
- abrachiaThe congenital absence of both arms
- abrachiocephaliaacephalobrachia; abrachiocephaly; a congenital absence of arms and head.
- abranchiataAn order of annelids, so called because the species composing it have no special organs of respiration, usually including Oligochaeta.
- acatalepsyIncomprehensibility of things; the doctrine held by the ancient skeptic philosophers, that human knowledge never amounts to certainty, but only to probability.
- acentricNot centered; without a centre.
- acerousDestitute of tentacles.
- achaenocarpAny dry indehiscent fruit.
- acheneA small, dry, indehiscent fruit, containing a single seed, as in the buttercup.
- achilousWithout a lip.
- achlamydateNot possessing a mantle.
- acholiaDeficiency or absence of bile.
- achromatizeTo deprive of colour; to make achromatic.
- Acœmitæa 5th century monastic order that organised themselves into sleep cyles to allow for a unbroken chain of devotion to be performed.
- acopicRelieving weariness; restorative.
- acrotismA lack or defect of the pulse.
- adermatoglyphiaThe absence of epidermal ridges (fingerprints and toeprints).
- adiaphoriststhe followers of Phillip Melancthon's doctrines of Lutheranism, especially regarding the Lutheran's adoption of the Interim of Charles V, Holy Roman Emperor.
- agnathAn agnathan.
- agnathaAn agnathan.
- agnathanBelonging or pertaining to the superclass Agnatha, the jawless vertebrates.
- agnathiaA birth defect in which the mandible is missing.
- agnathicJawless.
- agnomicalOf or pertaining to the absence of set purpose or intention.
- agnosisEpistemologically necessary lack of, indifference to, denial or shunning of knowledge; defective knowledge.
- agyriaThe smooth appearance of the brain in cases of lissencephaly.
- aphagiaThe condition of difficulty in swallowing.
- aphantasiaA condition where one does not possess a functioning "mind's eye" and cannot visualize imagery.
- apneusticOf or relating to apneusis.
- athymiaThe absence of emotion; morbid impassivity.
- azeotropeA mixture of two or more substances whose liquid and gaseous forms have the same composition (at a certain pressure); the substances cannot be separated by normal distillation.