unionidZool.|ˈjuːnɪəʊnɪd| [a. mod.L. Unionid-æ(seedef.), f. L. unio unio: see -id3.] A member of the Unionidæ, a large family of bi-valve molluscs typified by the genus Unio of freshwater mussels; a unio. 1861P. P. Carpenter in Rep. Smithsonian Instit. 1860, 263 The extreme forms of the Unionids.