An ultra-mobile PC, or ultra-mobile personal computer (UMPC), is a miniature version of a pen computer, a class of laptop whose specifications were launched by Microsoft and Intel in Spring 2006. Sony had already made a first attempt in this direction in 2004 with its Vaio U series, which was only sold in Asia. UMPCs are generally smaller than subnotebooks, have a TFT display measuring (diagonally) about 12.7 to 17.8 centimetres (5.0 to 7.0 in), are operated like tablet PCs using a touchscreen or a stylus, and can also have a physical keyboard. There is no clear boundary between subnotebooks and ultra-mobile PCs, but UMPCs commonly have major features not found in the common clamshell laptop design, such as small keys on either side of the screen, or a slide-out keyboard.
The first-generation UMPCs were simple PCs running Linux or an adapted version of Microsoft's tablet PC operating system. With the announcement of the UMPC, Microsoft dropped the licensing requirement that tablet PCs must support proximity sensing of the stylus, which Microsoft termed "hovering". Second-generation UMPCs used less electricity and therefore could be used for longer (up to five hours) and also had support for Windows Vista.