WordWeb Online

1. Happening or recurring at regular intervals
   "the periodic appearance of the seventeen-year locust";
   - periodical
    
2. Recurring or reappearing from time to time
   "periodic feelings of anxiety";
   - occasional

See also: annual, biannual, biennial, bimestrial, bimonthly, biweekly, biyearly, cyclic, daily, day-after-day, day-by-day, day-to-day, diurnal, fortnightly [Brit], half-hourly, half-yearly, hebdomadal, hebdomadary, hourly, midweekly, monthly, nightly, oscillating, oscillatory, semestral, semestrial, semiannual, semimonthly, semiweekly, sporadic, triennial, weekly, yearly

Antonym: nonperiodic

Encyclopedia: Periodic