<unk>  is an incomplete or arrested development of an organ or a part . It is descriptive of many medical conditions such as:
Underdeveloped breasts during puberty.
Underdeveloped testes in Klinefelter's syndrome.
Underdeveloped thymus in DiGeorge syndrome.
Underdeveloped labia majora in popliteal pterygium syndrome.
Underdeveloped cerebellum caused by mutation in the Reelin gene.
Underdeveloped tooth caused by oral pathology, such as Turner's<unk> .
Underdeveloped chambers of the heart in hypoplastic left heart syndrome and hypoplastic right heart syndrome
Underdeveloped optic nerve in optic nerve<unk> 
Underdeveloped sacrum in sacral agenesis
Underdeveloped facial muscle in asymmetric crying facies
Hypoplasia
Hyperplasia
Hemihypertrophy
Atrophy
Metaplasia
Dysplasia
Anaplasia
Tooth fusion