Hypoplasia

Hypoplasia (plural: hypoplasias) is a pathological term referring to the state of a tissue or organ which at the end of its developmental process has not fully formed (cf. aplasia in which the organ/tissue does not form at all). Thus, the term hypoplasia does not tend to be used for a later (post-developmental) decrease in size of an organ/tissue for which people tend to use terms such as atrophy or involution.

Interestingly therefore hypoplasia and hyperplasia are not opposites, despite the apparent antonymy of their prefixes. Hyperplasia is usually reserved for post-developmental enlargement of anatomical structures, e.g. muscle hypertrophy.