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$VersionNumber is a real number which gives the current Mathematica kernel version number, and increases in successive versions.
To find out if you are running under Version 3.0 or above, you can use the test TrueQ[$VersionNumber
A version with a particular number is typically derived from the same source code on all computer systems.
See the Mathematica book: Section 2.13.5.
See also: $ReleaseNumber.