pe·ri·od·ic 
(pîr′ē-ŏdĭk)
(pîr′ē-ŏdĭk)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.
pe′ri·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.
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.