'equal-petalled' originates from Modern English, specifically a compound of 'equal' and 'petalled', where 'equal' ultimately comes from Latin 'aequalis' meaning 'level, equal' and 'petalled' derives from 'petal' (from Greek 'petalon') meaning 'leaf or thin plate (of a flower)'.

'petal' entered English in the early 19th century from Greek 'petalon' via Neo-Latin/French botanical usage; 'equal' has roots in Latin 'aequalis' and passed into English via Old French and Middle English. The compound 'equal-petalled' is a Modern English descriptive formation used in botanical contexts.

Initially the components referred separately to 'equal' and 'petal'; over time the compound came to be used specifically in botanical description to denote flowers whose petals are uniform in size or shape.