Once upon a time...
... 72dpi (dots per inch) was considered to be the standard resolution for images on t'Interwebs. This is a fallacy, based upon the Macintosh default screen resolution (PCs used 96pdi).
72 dpi is a rough conversion from 72 points, which is how many points there are in an inch. There are 12 points in a pica, where we get ours standard font sizes from. A pica was the traditional form of measurement in typography the old school way. The 72 dpi for screen resolution was an early attempt on Macs to get a one to one correspondence between screen resolution and “old fashioned” print measuresments. 12 pixels on a mac screen should be the same as a 12pt font printed out. 72 pixels printed from a mac should be one inch. PCs have always been 96 dpi, hence the reason early desktop publishing and Mac, not PC, was a natural fit.
With the uptake of smartphones and other pixel-dense devices for consuming Web content, the pixels and DPI has become even fuzzier. From A List Apart:
"Things are changing for the pixel. The w3c currently defines the reference pixel as the standard for all pixel-based measurements. Now, instead of every pixel-based measurement being based on a hardware pixel it is based on an optical reference unit that might be twice the size of a hardware pixel. This new pixel should look exactly the same in all viewing situations."
Anyway, I've been using variations of pixelSEVENTY2 for over 10 years now, and it becomes a habit.
Chris, aka pixelSEVENTY2, aka pixel, aka PXL. February 2012