Basically, it's the seventh version of Windows. Count along with me:

    1.0:           Windows 1.0
    2.0:           Windows 2.0
    3.0:           Windows 3.0
    3.1:           Windows NT
    4.0:           Windows 95, 98, 98SE-second edition, ME-millenium edition
    5.0:           Windows 2000
    5.1:           Windows XP(Whistler)
    6.0:           Windows Vista(Longhorn)
    7.0:           Windows 7(Initially blackcomb,later vienna)

Some of them are given with their codename in bracket.