a-, an-

(Greek: prefix; no, absence of, without, lack of; not)

These prefixes are normally used with elements of Greek origin, a- is used before consonants and an- is used before vowels.

It affects the meanings of hundreds of words.

There are too many words that use these prefix elements to list all of them on this site; however, there are significant examples listed in this and the other units where they exist.

abyssopelagic (adjective), more abyssopelagic, most abyssopelagic
1. Relating to the region of deep water which excludes the ocean floor; floating in the ocean depths. Living in the oceanic water column at depths of between 4,000 and 6,000 meters [13,120 feet to 19,680 feet], seaward of the shelf-slope break: The marine biologists used specialized mechanical diving equipment to explore the abyssopelagic depths of the sea.
2. Of or relating to organisms or phenomena in midwater, but still at great depths: At the abyssopelagic depths of the sea, many of the creatures are blind.
acapnia (s) (noun), acapnias (pl)
1. Literally, the absence of carbon dioxide; however, the term is used to indicate less than the normal amount of carbon dioxide in the blood and tissues: The doctor was concerned about the acapnia in the patient’s blood.

Symptoms of acapnia include depressed respiration, giddiness, paresthesia (abnormal sensation like "burning"), cramps, involuntary contraction of the fingers, and occasionally convulsions.

Acapnea is a misspelling of acapnia.

2. Etymology: from Latin acapnos, "without smoke"; from Greek akapnos, a-, "not" + kapnos, "smoke" (carbon dioxide).
acardia (s) (noun), acardias (pl)
1. A congenital absence of a heart, sometimes occurring in the smaller member of conjoined twins when its partner monopolizes the placental blood supply: As indicated in this definition, some congenital twins have just one heart for both bodies; that is, one has a heart and the other one is acardia.

The veterinarian found out that the calf was born as an acardia and so it had no chance of even being born alive.

2. Having no heart: It hardly seems very romantic to describe Jane's ex-boyfriend as having acardia.
acarpous (adjective) (not comparable)
Without fruit, not bearing fruit; sterile: The fruit tree is strictly acarpous and will not bear any more fruit.
acatalepsia (s) (noun), acatalepsias (pl)
An abnormal inability to comprehend, or to understand: Some people have acatalepsia which is a form of mental deficiency.
acatalepsy (s) (noun) (no plural)
1. An ancient view that no more than probable knowledge is available to human beings: The explorers found the ancient documents about acatalepsy which clarified the Skeptical theories of knowledge.
2. The impossibility of complete discovery or comprehension; incomprehensibility: For Dave's cousin, advanced mathematics is an exercise in acatalepsy, reaching new degrees of not understanding.
3. The ancient doctrine that nothing can be known with certainty: Enrique's historical statement suggests that medical diagnostic or prognostic acatalepsy is inherently uncertain.
acataleptic (adjective), more acataleptic, most acataleptic
1. Conveying a condition of being deficient in understanding: Tracy's acataleptic mind simply could not understand the new concepts introduced in the advanced physics class.
2. Characterized by being unknowable or necessarily uncertain: As for the ancient explorers, when sailing across the ocean, they were experiencing acataleptic adventures.
acathectic (adjective) (not comparable)
A seldom used term for an abnormal release of secretions which is a reference to a failure of or to the inability to retain bodily secretions and excretions: Acathectic substances include saliva, mucus, tears, bile, or a hormone; sometimes, they cannot be secreted or discharged.
acathexia (s) (noun), acathexias (pl)
The inability to retain body secretions: When people live with acathexias, they usually exist with the abnormal losses of bodily secretions; for example, those that are separated from blood or which are formed by elements from the blood.
acathexis (s) (noun), acathexes (pl)
A mental disorder in which certain objects or ideas fail to provide an emotional response in the individual: When people have acathexes, they have an absence of psychic energy which causes the memory to have no recollection of subjects or things that were once significant and vital in their lives.
acathisia (s) (noun), acathisias (pl)
The inability to sit down because of the intense anxiety provoked by the thought of doing so: Wade, the patient of Dr. Jones, was an interesting example of someone suffering from acathisia as indicated by the fact that he was unable to relax and sit down in the doctor's office.

Acathisias may also apply to the inabilities that some people have of not being able to sit still and to other irritative, hyperkinetic symptoms that are sometimes seen as complications of neuroleptic therapies.

acaudal (adjective) (not comparable)
Pertaining to being without a tail or not having a tail-like appendage: There are acaudal lizards living in some desert areas.
acaudate (adjective) (not comparable)
1. Without, or lacking, a tail: The acaudate bobcat was seen leaping from rock to rock in the mountains.
2. Characterized by the congenital absence of a tail in an individual of a species that normally has this appendage: The birth of an acaudate foal was distressing because she would be unable to flick away the flies from her back end.
acaulescent (adjective), more acaulescent, most acaulescent
With no stem or with a stem that is very short: Olivia's favorite acaulescent plant is a popular thick-leaved houseplant that is native to South Africa and Asia which is called a "jade plant" because it has short, stemless leaves.
acausal (adjective) (not comparable)
A reference to not having a basis for any actions nor responses: The doctor's acausal medical report relieved Sidney's anxiety about his medical condition.