Pixels Per Inch
Dots Per Inch
In the context of a display, this is the number of pixels (dots) per inch, as measured along either the vertical or horizontal linear axis. (As pixels are almost always square, the number is generally the same both horizontally and vertically.)
This is also known as pixel density. More pixels per inch produces a sharper image with better detail and smoother lines and text. Displays with less pixels per inch can show "jagged" edges on things displayed, especially angled or curved lines, and text, due to the pixels being visible.
PPI refers only to the number of pixels per inch, not the display's total number of pixels, nor the total physical size of the display. But all three are related. Consider two displays that are the same physical size; if one has more total pixels (resolution) then it will necessarily also have greater PPI.
Note that PPI that is estimated (calculated based on the resolution and physical display size) is only as precise as the reported physical display size, which is often somewhat imprecise. (Phone Scoop estimates PPI this way.)