DPI
Dots per inch: the number of individual ink droplets a printer places along an inch of paper.
Measured in
dpi
In depth
An inkjet needs many droplets of a few fixed ink colours to reproduce one pixel's continuous tone, which is why droplet figures are an order of magnitude higher than image resolution. Raising the driver's dpi setting improves tonal smoothness slightly and uses more ink and time; it does not add image detail.
Examples in practice
A printer set to 2880 dpi is still reproducing an image sent to it at 300 ppi.