Wiktionary
n. (context ichthyology English) Either of two fins located on each side of a fish just behind the head.
WordNet
n. either of a pair of fins situated just behind the head in fishes that help control the direction of movement
Usage examples of "pectoral fin".
There was little other motion: an occasional correction of the apparently aimless course by the slight raising or lowering of a pectoral fin -- as a bird changes direction by dipping one wing and lifting the other.
Hikahi took Suessi's hand gently between her jaws, then did the same with Tsh't's left pectoral fin.
Then Nittaatsuq got what he thought was a brilliant idea and swam forward, thrusting his pectoral fin with the knife gripped clumsily in its fingers into the narrow crevice.
On its next breath the whale rolled and slapped a long pectoral fin on the surface, soaking Kona and throwing heavy spray over the console.
During his first fifteen minutes in the new tank, he savagely foreshortened a pectoral fin on another male orca and destroyed the floating basket through which he was supposed to slam-dunk beach balls.
There were, in the fish category, some Malabar rays, enormous gristly things, ten feet long, weighing 600 pounds, the pectoral fin triangular in the midst of a slightly humped back, the eyes fixed in the extremities of the face, beyond the head, and which floated like weft, and looked sometimes like an opaque shutter on our glass window.
Pitt kicked fiercely, twisting his body to one side as the shark flashed past, using its pectoral fin to push it away.