A Fin on the Back of a Fish

Puzzle 15 of Animation Pack

Question is: A Fin on the Back of a Fish

and the answer is: DORSAL

Word Meaning:

DORSAL — (anatomy, zootomy) With respect to, or concerning the side in which the backbone is located, or the analogous side of an invertebrate.

Definition source: Wiktionary (CC BY-SA 4.0)