A project, or set of projects, at MicrosoftCorporation, with varying scope at varying times; never shipped.  At various times, Cairo has been billed as:

* A "next-generation" version of Windows.  When Cairo was being hyped in the mid-90s, many assumed that NT 4.0 would be Cairo.  When NT 4.0 shipped and it amounted to little more than the WindowsNinetyFive UI on top of the NT 3.5 core, people started speculating that NT 5.0 would be Cairo.  NT 5 shipped, though renamed WindowsTwoThousand, and that ''still'' wasn't Cairo.  Nor was WindowsXp, WindowsTwoThousandAndThree.  Maybe WindowsLonghorn?

* A long-running "technology development" project, which prototypes and develops new technologies that are subsequently incorporated into products, but which is never shipped itself.

* An attempt by MicrosoftCorporation to develop an object based file system. (note that it was long-rumored that WindowsLonghorn will have similar technology, though as of late 2004 it's been dropped from there as well; see 
*** http://www.theregister.co.uk/2004/08/31/avalon_winfs_decoupled/).

A classic example of VaporWare.
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CategoryMicrosoft