Entry 1 sense Webster, 1913 Ascidian · As·cid·i·an · IPA /əˈsɪdi.ən/ 01 n. One of the Ascidioidea, or in a more general sense, one of the Tunicata. Also as an adj. 1. One of the Ascidioidea, or in a more general sense, one of the Tunicata. Also as an adj.(Zool.)