Antonyms of adj periodic

2 senses of periodic

Sense 1
periodic (vs. aperiodic), periodical -- (happening or recurring at regular intervals; "the periodic appearance of the seventeen-year locust")

aperiodic (vs. periodic), nonperiodic -- (not recurring at regular intervals)
        => noncyclic -- (not having repeated cycles)
        => nonoscillatory -- (not having periodic vibrations)

Sense 2
periodic, occasional -- (recurring or reappearing from time to time; "periodic feelings of anxiety")

INDIRECT (VIA sporadic) -> continual -- (occurring without interruption; chiefly restricted to what recurs regularly or frequently in a prolonged and closely spaced series; "the continual banging of the shutters")

Similarity of adj periodic

2 senses of periodic

Sense 1
periodic (vs. aperiodic), periodical -- (happening or recurring at regular intervals; "the periodic appearance of the seventeen-year locust")
       => cyclic -- (marked by repeated cycles)
       => oscillatory, oscillating -- (having periodic vibrations)
       => diurnal -- (having a daily cycle or occurring every day; "diurnal rotation of the heavens")
       => daily, day-to-day, day-by-day, day-after-day -- (of or belonging to or occurring every day; "daily routine"; "a daily paper")
       => nightly -- (happening every night; "nightly television now goes on until 3:00 or 4:00 a.m.")
       => weekly, hebdomadal, hebdomadary -- (of or occurring every seven days; "a weekly visit"; "weekly paper")
       => semiweekly, biweekly -- (occurring twice a week)
       => hourly -- (occurring every hour or payable by the hour; "hourly chimes"; "hourly pay")
       => half-hourly -- (occurring ever half hour)
       => fortnightly, biweekly -- (occurring every two weeks)
       => annual, yearly -- (occurring or payable every year; "an annual trip to Paris"; "yearly medical examinations"; "annual (or yearly) income")
       => semiannual, biannual, biyearly, half-yearly -- (occurring or payable twice each year)
       => biennial, biyearly -- (occurring every second year; "they met at biennial conventions")
       => triennial -- (occurring every third year or lasting 3 years)
       => monthly -- (of or occurring or payable every month; "monthly payments"; "the monthly newsletter")
       => bimonthly, bimestrial -- (occurring every two months; "bimonthly visits")
       => semimonthly, bimonthly -- (occurring twice a month; "a semimonthly publication")
       => semestral, semestrial -- (occurring every six months or during every period of six months)
       => midweekly -- (occurring during the middle of the week; "midweekly prayer meetings")

Sense 2
periodic, occasional -- (recurring or reappearing from time to time; "periodic feelings of anxiety")
       => sporadic (vs. continual) -- (recurring in scattered and irregular or unpredictable instances; "a city subjected to sporadic bombing raids")