
sporadisch
sporadic
Occurring occasionally, irregularly, or only in isolated cases; not continuously or systematically.
Er meldet sich nur sporadisch, meistens einmal im Monat.
He only gets in touch sporadically, usually once a month.
sporadic
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sporadic
Occurring occasionally, irregularly, or only in isolated cases; not continuously or systematically.
Er meldet sich nur sporadisch, meistens einmal im Monat.
He only gets in touch sporadically, usually once a month.
'Sporadisch' (sporadically) is very often used as an adverb to describe how an action is carried out: irregularly. It then usually appears directly with the verb.
Wir sehen uns nur sporadisch, da wir in verschiedenen Städten wohnen.
We only see each other sporadically because we live in different cities.
Common combinations are 'sporadische Kontrollen' (sporadic checks), 'sporadischer Kontakt' (sporadic contact), or 'sporadische Regenschauer' (sporadic rain showers). It always describes something that does not happen continuously or according to plan.
Es gab nur sporadische Proteste gegen das neue Gesetz.
There were only sporadic protests against the new law.
The word 'sporadisch' (sporadic) belongs more to written language or a somewhat more elevated style. It sounds more formal and precise.
Die Stromversorgung in der Region fällt sporadisch aus.
The power supply in the region fails sporadically.
rare / seldom
Selten and sporadisch can both express low frequency. Selten emphasizes that something does not happen often; sporadisch additionally highlights that it occurs sporadically and without a regular pattern.

Seit ihrem Umzug besucht sie uns nur noch sporadisch und ohne festen Rhythmus.
She has only visited us sporadically and without a regular schedule since she moved.
Seit ihrem Umzug besucht sie uns nur noch selten.
She has only visited us rarely since she moved.