Question

what is a flood myth ?

Answer 1

Any myth involving a flood. So Noah's Ark is a flood myth in Christian mythology (or religion, if it's your bag) but there are similar stories in other belief systems that involve some kind of great flood. Just think of flood myths as any story that tells of a large, usually global, flood.

Answer 2

There is, for instance, the Gilgamesh story, which is the precursor to (and probable source for) the Noah story --

http://en.wikipedia.org/wiki/Gilgamesh_flood_myth

But there are many others as well --

http://www.talkorigins.org/faqs/flood-myths.html

http://www.nwcreation.net/noahlegends.html