An inter-trial interval (ITI) is the time between separate trials (conditioning by presentation of stimuli) in behaviorist learning research. The ITI is usually measured at the beginning of a trial and lasts until the beginning of the following trial.
An example ITI could be seen in research conditioning mice to react to the color red. A red light is presented to the mice for 20 seconds and then food is given to them. Then 120 seconds later the red light is presented again. So the ITI for this experiment is 140 seconds.