The Merriam-Webster dictionary includes the following main entry:

graphic novel

Pronunciation: \ˈgra-fik \ \ˈnä-vəl \

Function: noun

: a fictional story that is presented in comic-strip format and published as a book.1 

Two words. A simple definition.

If only the use and execution of the term were as simple as Merriam-Webster's definition of it.

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Footnotes
1. Merriam-Webster Dictionary, Online ed., s.v. "Graphic Novels." http://www.merriam-webster.com/dictionary/graphic%20novel.