Dots Per Inch (dpi) or Pixels Per Inch? (ppi)

Long before there were computers or monitors or pixels, there were "dots per inch." Printers used this term referring to how many dots would be used to print. A higher dot amount per inch would be a higher resolution print.  While an every day magazine would print at 150 dpi, an art print might print at 400 dpi. It referred to printing plates and the films that were used to make the plates.

Somehow or other, people who had no idea what they were talking about, started using the printing term "dots per inch" to refer to "pixels per inch." While completely irrelevant to each other, since images have "pixels" not dots, the dots per inch term has stuck and been picked up by other people who do not know what they are saying and means the same thing as the term "pixels per inch."
Pixels per inch = Dots per inch.

We scan at 4,000 "pixels per inch".
We also scan at 4,000 "dots per inch".