Flan is generally used in reference to the Flan people, (sometimes called the "Flannae"), a race of humans of Oerth, the World of Greyhawk, though the term may also refer to the language and culture of said people. The Flan peoples were the first humans known to have settled the eastern portion of the continent of Oerik, the Flanaess, which is named for them.
There is an article about the Flan at Greyhawk Wiki.