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.
周期的 stresses a cyclical or repeating natural pattern, while 定期的 simply means occurring at set, scheduled intervals.
Compound of 周期 (cycle, period) + 的 (-like, -ic). The suffix 的 is a productive way to form adjectival nouns from nouns.