Noun
caudal peduncle (plural caudal peduncles)
(ichthyology) The narrow part of a fish's body to which the caudal or tail fin is attached. Its proportions, such as the length and breadth, and attached organs, such as spines of surgeonfishes or caudal keels and finlets of Scombridae often are important in descriptions of fish species.
After initially feeding on the whale caudal peduncle and fluke, the sharks would investigate the carcass by slowly swimming around it and mouthing several parts before selecting a blubber-rich area. Source: Internet
The caudal peduncle is absent and the caudal fin is reduced to a stiff rudder-like structure. Source: Internet