The fin on the back of an orca is called the dorsal fin.

If you mean a fin like a fish fin, there is a word, but mostlly each one is different; Lā (fin). Anal fin, kuaalo. Caudal fin, hiʻu. Dorsal fin, kualā. Pectoral fin, etc.

If you mean fin, like 'the end', it is pau'ana.

The dorsal fin is the fin on the back of a fish, for example the typical triangular fin on the back of a shark is its dorsal fin.

An adipose fin is a soft, fleshy fin found on a fish behind the dorsal fin and ahead of the caudal fin.

Soup fin is shark fin used in making shark fin soup.