Ultrabook is a standard defined by Intel for thin-and-light notebooks. Originally meant to provide a PC form factor to compete against low-power ARM-based slate computers such as the iPad, it's a refinement of existing ideas and designs in the laptop industry dating back at least as far as 2004.
Intel's standard includes strict requirements with regards to thickness, weight, battery life and other aspects that a laptop must meet in order to be branded as an Ultrabook. While these standards change slightly with each new mobile platform that Intel releases, Ultrabooks are generally required to use ultra-low voltage Intel processors, be no thicker than 21 mm (0.82 inches) and no heavier than 1.5 kg (3.3 lbs), as well as be able to run on battery for at least 5 hours and resume from hibernation in no more than 7 seconds.
As of 2012, all major computer OEMs have at least one, and in most cases several, Ultrabooks in their lineup, with screen sizes ranging from 11.6 to 15 inches.