a stroke of luck
"Winning the lottery was nothing more than a lucky fluke for us."
a barb on a harpoon or arrow
"The hunter checked to make sure each fluke on his harpoons was sharp before heading out to sea."
either of the two lobes of the tail of a cetacean
"The whale's powerful flukes pushed it smoothly toward the surface to breathe."
parasitic flatworms having external suckers for attaching to a host
"The surgeon carefully removed the fluke from the patient's liver after it had burrowed there and used its external suckers to cling tightly to the tissue."
A lucky or improbable occurrence, with the implication that the occurrence could not be repeated.
"The massive humpback whale raised its flukes high out of the water before diving deep into the ocean."
A flounder.
Either of the two lobes of a whale's or similar creature's tail.
To obtain a successful outcome by pure chance.
"Winning the lottery was nothing more than a fluke, as I never expected to buy that single ticket."
A surname.
"My neighbor, Mr. Fluke, was at the door asking about the party tonight."