back in the early 90s, when lighter machines first started coming out it was based on size.
notebooks tended to be stuff like less than 2" thick. back then laptops were like 2.5" thick.
basically a laptop would be what is a 3 spindle now, and a 2 spindle would be a notebook. 1 spindle being a subnotebook. And a really thin subnotebook (think toshiba portege 2000 as ultralight, and a sony viao srx77 as a subnotebook) would be a ultralight (under 3 pounds)