Microsoft has taken the term and made it their own, of course. They refer to their high-CPU, high-RAM version of 2K as "datacenter".
A data center is, in short, a computer room. The word Data Center typically refers to a larger computer room, at least a couple of thousand square feet. They usually have a raised floor (for cooling purposes and to run cables, if your city permit codes allow it). Generally, a data center is backed up by a large-scale UPS (50+X the size of your average little APC UPS) for power protection and that's often backed up by a generator. Also included are large-scale AC systems and all kinds of fancy electrical (PDU's, etc.) Data centers are fun stuff to design and built.
Then again.. Any computer room can really be called a data center, but a 200SF closet with a coupla servers and a switch doesn't really cut it, IMHO.
- G