[Tagbert]

Pixels have pitch, which is the distance between pixels. That is what is usually meant when talking about px as a measurement. It is analogous to dpi or ppi or the metric version.

[ethmarks]

Does the pitch describe the distance between the edges of the pixels or between the centers?

[aptly_yclept]

To expand on what the previous said, here's a link to a seminal paper on pixels - "A pixel is not a little square" by Alvy Ray Smith. https://alvyray.com/Memos/CG/Microsoft/6_pixel.pdf

[Tagbert]

center to center.