In particle and nuclear detectors the dead time is the time after each event detected by the detector in which the detector is not able to reveal another event if it happens.
In this time the detector is as it was dead or frozen or paralysed. Even if it is really live it is not able to count. This may be due both to real event and spourius event. In some case the detector is not even sensitive to another event (for exampke in a spark chamber until the potential between the plate is not recover above an enough high value) in other case the detector is sensitive but is not eable to giving out an information or the information is not trustable.
An easy example in every day life is given by the screen of Cathode ray tube television set. When a beam arrived on the screen and light the pixel on then it remains on for a fraction of second even if the beam is stopped. This effect and the persistence effefct in the retina of human eyes make possible to view a complete image even if is compund of points that are light on at little different time.
From the total time a detector is running, the dead time nust be subtract to obtain the live time of the experiment