Ultrabook™ is an Intel trademark that describes a laptop specification. Notably, ultrabooks have an Intel CPU, SSD storage, and a thin unibody frame. They have a maximum height (when closed) and therefore do not have an optical drive, such as a DVD player.
The initial Ultrabook spec was announced in 2011, which included an Intel "Sandy Bridge" processor, a maximum height of 21mm, 5-hour battery life, a maximum 7-second resume-from-hiberation time, and Intel's Anti-Theft Technology. The 2012 update required a minimum "Ivy Bridge" Intel processor, USB 3.0 or Thunderbolt ports, and a minimum data transfer rate of 80 MBps. The 2013 Ultrabook update increased the minimum processor to a "Haswell" Intel processor and required 6 hours of high-definition video playback.
Since laptops have gotten thinner and most no longer have optical drives, Ultrabooks are not as exclusive as they once were. However, if you are shopping for a new laptop, it can still be helpful to check if the laptop meets the requirements of an Ultrabook.