A pixel is not a little square and also not an unit of length and area
Once in a while I see two articles pop up on https://news.ycombinator.com/ A pixel is not a little square: http://alvyray.com/Memos/CG/Microsoft/6_pixel.pdf and Pixel is a unit of length and area: https://www.nayuki.io/page/pixel-is-a-unit-of-length-and-area
I think the first article is right, the second is not. A pixel is a sample of color / energy at a location in a grid. But we have no idea about the size of this grid.
If we have, for example, an image of 400x300 pixels we can only say we have a grid of samples of 400 columns and 300 rows. We don't know if the image is 1mm or 10cm in size, we don't even know if it is rectangular or square.
Yes, most of the time it is assumed that all samples were taken at an equal distance. So the above example would be rectangular in shape. But if we sampled columns at a shorter distance it could also be a square image.
The point of this post is: it can help to think about pixels as samples without size. For example anti-aliasing can be seen as adding more samples without changing the display size of an image.
A pixel is not a little square.