tailfin

Concise Oxford English Dictionary © 2008 Oxford University Press:
tail fin

▶noun

• 1 Zoology a fin at the posterior extremity of a fish's body, typically continuous with the tail.

• 2 a projecting vertical surface on the tail of an aircraft, housing the rudder.

• 3 an upswept projection on each rear corner of a car, popular in the 1950s.