The frequency at which the balance wheel and spring oscillate to and fro (back and forth).

Common mechanical wristwatch frequencies:

18,000 VPH or 2.5Hz
21,600 VPH or 3Hz
28,800 VPH or 4Hz
36,000 VPH or 5Hz
43,200 VPH or 6Hz
72,000 VPH or 10Hz

The balance of a mechanical watch generally makes 5 or 6 vibrations per second (18,000-21,600 per hour.)

A high-frequency watch makes 8-10 vibrations per second (25,200/28,800/36000/43,200 per hour).

VPH “Vibrations per hour” is also called BPH “beats per hour.”