Periodic can refer to two different concepts.
For circuit mechanisms, periodic is the opposite of p1 or stable. Periodic circuits necessarily contain oscillators, and therefore they can generally only accept input signals that are synchronized to the combined period of those oscillators (but see universal regulator).
For signal streams, periodic means that signals will only be present in the stream at one out of every n ticks, where n is the period of the stream. In an intermittent periodic stream there may be gaps, so that signals do not always appear at every nth tick. However, if a signal does appear, its distance measured in ticks from previous and future signals will always be an exact multiple of n.