Jitter is theoretically defined as the short term non-cumulative variations of significant instants of a digital signal from its ideal position in time. 

Two common methods of measuring jitter are 

1) Period jitter with a digital storage oscilloscope (DSO) or time interval analyzer (TIA). This measurement is usually made in pSec. 

2) Phase jitter measurements are made over a specific frequency band and recalculated to the time domain. This method is used typically for SONET equipment jitter specifications. 

These measurements are usually made in pSec RMS in ___KHz to __MHz.