periodic

/ˌˈpɪriˌɑdɪk/

/piriˈɒdɪk/

Something periodic happens only occasionally, after a "period" of time. Lunch happens every day, after 4th period, so it's safe to say it's periodic. Something periodic doesn't have to happen at regular intervals, though.

The word "period" is the key to the meaning of periodic, because something periodic can happen only after a period of time. Periodic visits to the vet are a good way to make sure that your pet stays healthy. Why not just use "occasional," you ask. Good question. Periodic implies that the thing will continue to happen over and over—periodically. Like comets and victories by the Yankees. These things are reliable, if periodic.

Definitions of periodic

1. adjective

   happening or recurring at regular intervals
   “the periodic appearance of the seventeen-year locust”

   synonyms: periodical

   cyclic

   marked by repeated cycles

   oscillating, oscillatory

   having periodic vibrations

   diurnal

   having a daily cycle or occurring every day

   daily, day-after-day, day-by-day, day-to-day

   of or belonging to or occurring every day

   nightly

   happening every night

   hebdomadal, hebdomadary, weekly

   of or occurring every seven days

   biweekly, semiweekly

   occurring twice a week

   hourly

   occurring every hour or payable by the hour

   half-hourly

   occurring every half hour

   biweekly, fortnightly

   occurring every two weeks

   annual, yearly

   occurring or payable every year

   biannual, biyearly, half-yearly, semiannual

   occurring or payable twice each year

   biennial, biyearly

   occurring every second year

   triennial

   occurring every third year or lasting three years

   monthly

   of or occurring or payable every month

   bimestrial, bimonthly

   occurring every two months

   bimonthly, semimonthly

   occurring twice a month

   semestral, semestrial

   occurring every six months or during every period of six months

   midweekly

   occurring during the middle of the week

   antonyms:

   aperiodic

   not recurring at regular intervals

   noncyclic

   not having repeated cycles

   nonoscillatory

   not having periodic vibrations
2. adjective

   recurring or reappearing from time to time
   “periodic feelings of anxiety”

   synonyms: occasional

   sporadic

   recurring in scattered and irregular or unpredictable instances