A unit interval (UI) is the time taken in a data stream by each subsequent pulse (or symbol).
For example, in a serial line with a baud rate of 2.5 Gbit/s, a unit interval is 1/(2.5 Gbit/s) = 0.4 ns/baud.
The widespread use of UI in jitter measurements comes from the need to apply the same requirements or results to cases of different symbol rates.
This can be done when the phenomena investigated are not independent from the symbol duration time but closely related to it.
For example, UI is used to measure timing jitter in serial communications or in on-chip clock distributions.