Semantic zoom is a feature of Microsoft's Windows 8 operating system. It allows a user to activate a secondary display by using two fingers on the screen. The semantic zoom feature allows pinching an on-screen element or reverse pinching it to view the full display or organize on-screen elements.
Windows 8 is expected to be launched in January 2012. The developer version of Windows 8 is available on Microsoft’s website for download.
Techopedia explains Semantic Zoom
Semantic zoom's pinch-to-zoom gesture lets a user zoom out a group of tiles on a single screen. Microsoft is replicating the pinch gesture of touch screen phones to laptops with the semantic zoom feature in Windows 8. With just a pinch, a user can scroll, expand, shrink, hide, rename and perform other activities on screen.
The semantic zoom feature can be visualized with Google Maps, in which a user zooms into a particular location. However, the difference in Windows 8 is that the user can view a full display or shrink the elements to fit into the available screen size with two fingers.
This definition was written in the context of Windows 8