Entry 1 sense Webster, 1913 Abactinal /(ăb*ăk"tĭ*nal)/ · Ab·ac·ti·nal · IPA /æˈbæk.tə.nl̩/ 01 a. Pertaining to the surface or end opposite to the mouth in a radiate animal; -- opposed to actinal. 1. Pertaining to the surface or end opposite to the mouth in a radiate animal; -- opposed to actinal.(Zool.)