I agree with GrandSpleen. Just because lotr is techically one big novel split into 6 books divided in 3 novels doesn't mean it's not a trilogy. At least in my case long before the movies, lotr had always been a trilogy.
But it's not a trilogy. The volumes (i.e. FotR, TT, RotK) don't stand alone as individual narratives. One must read the entire Novel to get the story. Other trilogies (Star Wars, Indiana Jones, Iron Man, etc.) have installments that are episodic and do stand alone (Jackson's movies do stand alone as well, unlike the three volumes) and also create a larger narrative, that makes them trilogies.
EDIT: from Wikipedia:
One of the most popular "trilogies" of fantasy books, The Lord of the Rings by J. R. R. Tolkien, is not a trilogy, though it is often referred to as such. Tolkien regarded it as a single work and divided it into a prologue, six books, and five appendices. Because of post-World War II paper shortages, it was originally published in three volumes. It is still most commonly sold as three volumes, but has also been published in one-volume and seven-volume editions (six books and the appendices).
Fair enough. That doesn't mean though that it was particularly because of the movies that it came to be considered a trilogy.
Edited by Gizlivadi