The definition of "Ultrabook" is very specific, and did not apply to ultraportables of the past. It takes into account weight, thickness, battery life, resume from hibernate time, and speed of the storage and processors along with including some specific Intel features. It's a term Intel made similar to Centrino in the past. In other words, mostly a marketing brand/term.
Technically, the MacBook Air is not an Ultrabook either. And Apple has made ultraportables before the Air, such as the 12 inch Powerbook G4 and the Powerbook Duo.