fin
noun[L. pinna or penna. The sense is probably a shoot, or it is from diminishing. See Fine.]
The fin of a fish consists of a membrane supported by rays, or little bony or cartilaginous ossicles. The fins of fish serve to keep their bodies upright, and to prevent wavering or vacillation. The fins, except the caudal, do not assist in progressive motion; the tail being the instrument of swimming.