frequency is the number of times something happens per second (pulse frequency - the number of pulses per second), the units for frequency is Hertz (hz).

The period of a waveform is the the time required for one complete cycle. To convert from something measured in Seconds to something that is N per second, you simply take the inverse (i.e. divide it into 1). So a period of 5 ms (0.005 seconds), has a frequency of 1 / 0.005 or 200 hz. You can convert back too 1 / 200 hz = 0.005 sec

^^^ Perfect!

Sometime "*x*" is used instead.

But generally N is used for SI terms.