XENIX was a Microsoft port of V7 UNIX to Intel 8086 processors and the IBM-PC, among other 8 and 16-bit systems. Most of AT&T's System III was merged in.
The name XENIX came from a provision in AT&T's early UNIX source license agreements, namely that a product could not be resold under the name UNIX. It appears to be meaningless.
XENIX was a smash hit, and at one time, was by far the most popular UNIX system in the world. As its popularity waned, it was sold to SCO, who continued to merge AT&T code into it and sold it as variously XENIX/286, SCO XENIX, SCO XENIX System V, SCO UNIX, SCO OpenDesktop, and finaly SCO Openserver. SCO UnixWare is an entirely different product and codebase, originally developed by Univel.
Rumor has it that Microsoft continued to use XENIX and common UNIX tools such as nroff and troff internally until 1989 or 1990, but I cannot find corroborating evidence.
The prospective XENIX user should be warned, it was a primitive system, even by the standards of the day. XENIX and XENIX/286 did not support memory protection due to hardware limitations. No product sold under the name SCO UNIX ever included TCP/IP. Finally, in the name of backwards compatibility, XENIX and its derivatives have always been subtly different in annoying ways from their UNIX brethren...
To be perfectly clear, SCO UNIX is a XENIX derivative, not the other way around.