'isobranchial' originates from Greek elements, specifically 'iso-' from Greek 'ἴσος' where 'iso-' meant 'equal', and 'branchia' from Greek 'βράγχια' meaning 'gills'.

'isobranchial' was formed in New Latin/Neo-Latin from the Greek elements 'isos' + 'branchia' (used in scientific/medical Latin as 'isobranchialis') and then entered modern English in specialized zoological/anatomical use as 'isobranchial'.

Initially it meant 'having equal gills' in a literal, descriptive anatomical sense; over time it has remained a specialized term with the same technical meaning in zoology and comparative anatomy.