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
    marked by repeated cycles
    oscillating, oscillatory
    having periodic vibrations
    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
    happening every night
    hebdomadal, hebdomadary, weekly
    of or occurring every seven days
    biweekly, semiweekly
    occurring twice a week
    occurring every hour or payable by the hour
    occurring ever 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
    occurring every third year or lasting 3 years
    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
    occurring during the middle of the week
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    aperiodic, nonperiodic
    not recurring at regular intervals
    not having repeated cycles
    not having periodic vibrations
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  2. adjective
    recurring or reappearing from time to time
    periodic feelings of anxiety”
    synonyms: occasional
    recurring in scattered and irregular or unpredictable instances
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