From The Free On-line Dictionary of Computing (8 July 2008) [foldoc]:
DB2
<database> A relational database from IBM.
When running under IBM's MVS (Multiple Virtual Storage)
operating system, DB2 is implemented on top of VSAM and
uses its underlying data structures.
DB2, later called "DB2 Universal DataBase", also runs under
windows NT, AIX, Solaris and, most recently, Linux.
[Details? Was there a "DB1"?]
(1999-02-01)