Definition of "fin"

fin¹

/fɪn/

noun
plural fins
1. (ichthyology) One of the appendages of a fish, used to propel itself and to manoeuvre/maneuver.
  Quotations
  Then he commenced to talk, really talk. and inside of two flaps of a herring's fin he had me mesmerized, like Eben Holt's boy at the town hall show. He talked about the ills of humanity, and the glories of health and Nature and service and land knows what all.
  1913, Joseph C. Lincoln, chapter 4, in Mr. Pratt's Patients
2. A similar appendage of a cetacean or other marine animal.
3. A thin, rigid component of an aircraft, extending from the fuselage and used to stabilise and steer the aircraft.
4. A similar structure on the tail of a bomb, used to help keep it on course.
5. (surfing) A similar structure on the bottom of a surfboard, used to help steer it.
6. A hairstyle, resembling the fin of a fish, in which the hair is combed and set into a vertical ridge along the top of the head from about the crown to the forehead.
7. A device worn by divers and swimmers on their feet.
8. An extending part on a surface of a radiator, engine, heatsink, etc., used to facilitate cooling.
9. A sharp raised edge (generally in concrete) capable of damaging a roof membrane or vapor retarder.
10. (nautical) The conning tower of a submarine.
verb
third-person singular simple present fins, present participle finning, simple past and past participle finned
1. (transitive) To cut the fins from a fish, shark, etc.
2. (intransitive) (Of a fish) to swim with the dorsal fin above the surface of the water.
  Quotations
  When you spot him finning just under the surface, you move up quietly and present [...] bait, usually a squid.
  1983, The Fisherman Who Laughed, page 54
3. (intransitive) To swim in the manner of a fish.
4. (transitive) To provide (a motor vehicle etc) with fins.

fin²

/fɪn/

noun
plural fins
1. (UK, formerly Australia, slang) a five-pound (£5) note; the sum of five pounds.
2. (US, slang, dated) a five-dollar bill; the sum of five dollars.

fin³

/fɪn/

noun
plural fins
1. (film, television, road signs) The finish, the end