Absence is simply defined as "Not being there."
Aplasia is the medical term meaning absence of development.
Asplenia is the medical term meaning absence of the spleen.
Athelia means absence of a nipple.
presence
Athelia is the medical term meaning absence of nipples.
Aphakia is the medical term meaning absence of the lens. This is often a result of cataract surgery.
Anhydrosis or anhidrosis, either spelling, is the medical term meaning an abnormal condition of an absence of sweat or perspiration.
arrhythmia
The medical term for absence of the external ear is "anotia." This condition involves complete absence of the external ear.
According to internetslang.com, it means "leave of absence". :)
anhydrous
Anencephaly.