I know what you're thinking: "What kind of a question is that? Everyone knows what a novel is."
That's not entirely true though, especially considering that the form of the novel has changed over the past 1,000 years from the publication of its earliest antecedents: the Sanskrit "novel" Dashakumaracharita by Dandin (6th/7th Century), the 7th Century tale Kādambari by Bāṇabhaṭṭa, and the 11th Century story The Tale of Genji by Murasaki Shikibu.
We know that a "novel" in its current form is defined as a lengthy fictional narrative written in prose. How long that narrative is comes under debate depending on publishing house, who you listen to, the phases of the moon, etc. Everyone does agree that the novel must be fictional, and it must be written in the prose style -- if it's written in poetic form then it becomes an extraordinarily long poem.
Of course, then you get into novels that include sections written in verse (Byron's Don Juan is a good example) and things start getting sticky. Despite this though, the novel became almost solely written in prose because of the simple nature of translating prose from one language to another. And the fact that it's easier to read a prose story in silence rather than one in verse ... well you can see why prose became the preference.
There were also a few works, such as Chaucer's The Canterbury Tales, that we'd consider a short story collection today rather than a novel. However, since the "novel" was still a new concept in England when Chaucer was writing, then we can excuse the Tales for not following convention.
A rather lengthy article at Wikipedia on this selfsame topic goes into exhaustive detail about the history and development of the novel, more than I'm going to go into here simply because I'm sure y'all are getting bored by now. To be honest, this topic sort of got away from me. So I'm turning it around to you: is there any novel you've read recently that you felt didn't classify as a novel?