na-adjective
Describes events, phenomena, or patterns that repeat at regular intervals. Used in both technical and everyday contexts.
この現象は周期的に起こる。
This phenomenon occurs periodically.
株価には周期的な変動が見られる。
Stock prices show cyclical fluctuations.
Compound of 周期 (cycle, period) + 的 (-like, -ic). The suffix 的 is a productive way to form adjectival nouns from nouns.