Laughing at most of the responses here, especially since the only one that even comes CLOSE to being right is Jedaisouls'. It's a factor of resolution vs physical size, and screens come in all sorts of different ratios of same.
There is no fixed value across devices, saying "72" or "96" is ignorant nonsense... Let's do some math based on some displays I happen to have access to here.
My current 17" laptop -- visible width 15 inches, resolution 1920x1080
Pixels Per Inch: 128
Old 17" Laptop -- visible width 14.25 inches, resolution 1440x900
Pixels Per Inch: 101
MSI Wind U123 Netbook -- visible width 8.75 inches, resolution 1024x600
Pixels Per Inch: 117
My primary workstation 27" IPS -- visible width 23.25", resolution 2560x1440
Pixels Per Inch: 110
Secondary workstation 24" LCD's (one left, one right) -- visible width 20.5", resolution 1920x1200
Pixels Per Inch: 93.6
Workbench 17" LCD -- visible width 13", resolution 1280x1024
Pixels Per Inch: 78.77
14" Junk VGA CRT -- Visible width 11",
1024x768 (what I'd run) = 93dpi
800x600 (what "normal" people run) = 72dpi
21" Viewsonic CRT -- visible width 17.8", resolution 1600x1200
Pixels Per Inch: 89.9
4.5" Cubot GT99 (android phone) -- visible 'long' way 4", resolution 1280x720
Pixels Per Inch: 320
42" Sanyo TV -- visible width 36.5", resolution 1920x1080
Pixels Per Inch: 52.6
Get the idea? (I may have gone overboard here)
Of course with 4k displays (4096x2160) displays coming down the pipe, interface scaling is going to be even more important, which is why declaring fonts or layouts in pixels is shortsighted idiotic nonsense; unless you want people to pretty much HAVE to rely on zoom to use your sites, which of course is why fixed width layouts are idiotic nonsense, since when you zoom those they don't adjust to fit the display resulting in side-scrolling or worse, a broken layout.
Again, guidelines like the WCAG exist for a reason.