Periodic signals (signal periodic) signals that resembles the format of all sa signals the original formats of the period, which means an up and down continuously alternating signal.
Signal is periodic (periodic signal) is a sign of a form of the original pattern repeats every period. Which means that the signal alternately up and down indefinitely.
The signal has a period (periodic signal) is a signal characteristic of original signal it every time period. Which means there are up and down of the signal switch and so on.