parts per million (ppm)
Parts per million (ppm) means "parts per million", i.e. `10^-6` or 0.000 001. It is a quantity used to indicate changes in electronic parameters, accuracies, stabilities, concentrations and mixing ratios.
A quartz oscillator with a frequency stability of 5 ppm has a maximum frequency deviation of 0.000 005 ppm, which is, related to an oscillator frequency of, for example, 1 MHz, only 5 Hz.