means Happening at regular, recurring intervals; occurring again and again in a predictable cycle.

from From the Greek 'periodos' — 'peri-' (around) plus 'hodos' (a way or road) — literally a going-around, a journey that loops back to its start. The Greeks used it for cycles of time and the returning paths of heavenly bodies; it traveled through Latin 'periodus' and into English, where by extension a 'period' became any stretch of time that comes round again, and 'periodic' the adjective for whatever keeps making that same circuit.