It seems it comes down to word count-
Novel- over 40,000 words
Novella- 17,500 to 40,000 words
Novelette- 7,500 to 17,500 words
Short story- under 7,500 words
(From Answers.com)
Tiger is about to nose over into novelette territory. It may do so today when I find somewhere to sit down with the laptop.