Time-hopping (TH) is a communications signal technique which can be used to achieve anti-jamming (AJ) or low probability of intercept (LPI).
To achieve LPI, the transmission time is changed randomly by varying the period and duty cycle of the pulse (carrier) using a pseudo-random sequence.
The transmitted signal will then have intermittent start and stop times.
Although often used to form hybrid spread-spectrum (SS) systems, TH is strictly speaking a non-SS technique.
Spreading of the spectrum is caused by other factors associated with TH, such as using pulses with low duty cycle having a wide frequency response.