'atresia' originates from Medical Latin/Modern Latin, ultimately from Greek 'a-' (not) + 'trēsis' meaning 'perforation' or 'opening'.

'atresia' came into English from New/Medical Latin 'atresia', which in turn was formed from Greek roots: the privative prefix 'a-' + a form related to 'trēsis' (opening, perforation). Over time the term was adopted in medical Latin and then into English usage in clinical contexts.

Initially it literally meant 'absence of a perforation/opening'; over time it became the technical medical term for a congenital absence or closure (or severe narrowing) of a normal body opening or tubular organ.