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pe·ri·od·ic (pîrē-ŏdĭk)
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adj.
1. Having or marked by repeated cycles: periodic oscillations.
2. Happening or appearing regularly or from time to time: periodic dental checkups; periodic rain showers.
3. Characterized by periodic sentences.

peri·odi·cal·ly adv.

Synonyms: periodic, sporadic, intermittent, occasional
These adjectives all mean recurring or reappearing now and then. Something periodic occurs at regular or unsurprising intervals: periodic disruptions in electricity. Sporadic implies scattered, irregular, unpredictable, or isolated instances: spies who could manage only sporadic contact. Intermittent describes something that stops and starts, often with no discernable pattern: heard intermittent gunfire in the distance. What is occasional happens randomly and irregularly: a retired celebrity still making occasional appearances in public.

The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2017 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
 

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