Abdominal \Ab*dom"i*nal\, n.; E. pl. {Abdominals}, L. pl. {Abdominales}. A fish of the group Abdominales. Abdominales \Ab*dom`i*na"les\, n. pl. [NL., masc. pl.] (Zo["o]l.) A group including the greater part of fresh-water fishes, and many marine ones, having the ventral fins under the abdomen behind the pectorals.