What is narrative?

My one-word definition? History.

Narrative is the chronological sequence of events in a text. In the case of a traditional (printed) text, it's a fixed sequence that can't be changed, except possibly by the author. In some printed texts, like the Choose Your Own Adventure novels, it's not a fixed sequence because you're given options as you read through the story, but it still progresses through one specific chronology.
In the case of hypertexts, and the interactivity that comes with most of them, there may be no intended sequence except for the one that is chosen by the reader. That's the essential difference between traditional texts and hypertexts, as Aarseth points out. The reader of a traditional text is passive and powerless to change the course of the narrative. In hypertexts, there are many, many combinations available to the reader, and therefore, many different possible outcomes for the sequence of the narrative.
In the case of role-playing games, the narrative is almost entirely created by the reader, beginning with the creation of a character, and then continuing with that character's experiences through the gameplay setting. Still, even though there may be millions of different possibilities, there is ultimately only one narrative that that single character can have. So I say that narrative is just history, just a sequence of events, given meaning by the order they occur in.