DPI
Also called CPI · dots per inch · counts per inch · mouse sensitivity
DPI (more accurately CPI) is how many position counts a mouse sensor reports for each inch you move it. Higher DPI moves the cursor further for the same hand movement.
A web page only sees cursor movement after the operating system has applied its own speed setting, so no browser test can read the true DPI of a mouse.
In practice
DPI sets how far the cursor travels for each inch of hand movement. Higher isn’t better on its own. Most people settle somewhere between 400 and 1600 with the operating system speed left at its default. A very high DPI paired with a low system speed can make the cursor feel jumpy, because small movements of the sensor are amplified and then scaled down.
How to check
No web page can read a mouse’s real DPI, because browsers only see movement after the operating system has applied its own speed setting. Check the DPI in the mouse maker’s software or on its DPI indicator light. To compare two settings, move the mouse a fixed distance across the desk and see how far the cursor travels.