Serial interval is
The gap in the time interval between the onset of primary case and the secondary case is known as serial interval. By collecting information about a whole series of such onset we get a distribution of secondary cases from which we can guess the incubation period of disease ( refer pg 100, park 23 rd edition )
Interval between primary and secondary case
Interval between first case and last case
Interval between primary case and all subsequent cases multiplied by secondary attack rate
Interval between incubation period and gestational period
Epidemiology