It'd be more accurate and illuminating to say that a px is an angle.
The reference pixel is the visual angle of one pixel on a device with a pixel density of 96dpi and a distance from the reader of an arm’s length. For a nominal arm’s length of 28 inches, the visual angle is therefore about 0.0213 degrees. For reading at arm’s length, 1px thus corresponds to about 0.26 mm (1/96 inch).
- https://www.w3.org/TR/css-values-4/#absolute-lengths
This explains why 1px is small on phones and large on televisions; the physical size varies by expected reader distance. The angle tries to stay constant, though in practice it gets rounded to the nearest multiple of the physical pixel size.
The reference pixel is the visual angle of one pixel on a device with a pixel density of 96dpi and a distance from the reader of an arm’s length. For a nominal arm’s length of 28 inches, the visual angle is therefore about 0.0213 degrees. For reading at arm’s length, 1px thus corresponds to about 0.26 mm (1/96 inch). - https://www.w3.org/TR/css-values-4/#absolute-lengths
This explains why 1px is small on phones and large on televisions; the physical size varies by expected reader distance. The angle tries to stay constant, though in practice it gets rounded to the nearest multiple of the physical pixel size.