ntpd and ppm

Hi,

I noticed when: ntpd logs time adjustments in: /var/log/daemon it uses a
unit of "ppm":

Jun 22 23:22:20 server ntpd[45813]: adjusting clock frequency by
-1.127600 to 0.056400ppm

I checked: man ntpd and: man 2 adjfreq, and while: man 2 adjfreq
mentions the same unit - "ppm" - it doesn't explain what that means.

What does "ppm" stand for ?

Thanks,

- J