inverted nipple; retracted nipple
Anatomical term for a nipple that is pulled inward rather than protruding. Used in medical and everyday contexts.
See also: 陥没乳頭
An inverted nipple can be present from birth or can develop later in life.
Synonymous medical term; 乳頭 is a more formal anatomical word for nipple, while 乳首 is the common everyday term.
Compound of 陥没 (かんぼつ, 'caving in, depression') and 乳首 (ちくび, 'nipple'). The exact historical derivation is uncertain, but the term transparently describes the condition.