1. Having or marked by repeated cycles.
2. Happening or appearing at regular intervals.
3. Recurring or reappearing from time to time; intermittent.
4. Characterized by periodic sentences.

Synonyms: These adjectives all mean recurring or reappearing now and then. Something periodic occurs at regular or at least generally predictable intervals: periodic feelings of anxiety.
Sporadic implies scattered, irregular, unpredictable, or isolated instances: sporadic bombing raids.
Intermittent describes something that stops and starts at intervals: intermittent rain showers.
What is occasional happens at random and irregularly: occasional outbursts of temper.
Something fitful occurs in spells and often abruptly: fitful bursts of energy.

Usage Note: In technical use, periodic means "at regular or predictable intervals," as in the Periodic Table of the Elements. Often, however, periodic is used to mean "occasional, intermittent." This usage can be confusing for readers who are accustomed to the narrower sense of the word. Thus the writer who says Parker's losses at the track were not covered by his periodic wins invites the (most likely unintended) inference that Parker has a system that enables him to win at regular intervals. The ambiguity can be avoided here by using occasional instead.