Vacuum strength is rated in terms of?

The goodness of a vacuum is still measured in archaic units, millimetres of Mercury.

mmHg.

The development of vacuum pumps was paralleled by the development of better vacuum gauges, but unfortunately not in lock-step.

So folk invented a better pump, but the gauge showed not much improvement. Then soon after, a better gauge indicated a better vacuum had in fact been achieved! And next, an even better gauge was invented. And so on.

In mass spectrometry, vacuum of the order of 10-16mm is achieved by using ion pumps. In which an electrical plasma is used to purge the vacuum by using electromagnetic fields to move the odd atoms left.

(After several other pumps had reached their peak of performance in turn.)