adventitious
/ˌæd.vənˈtɪʃ.əs/
/ˌæd.vənˈtɪʃ.əs/
adjective
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From an external source; not innate or inherent, foreign.
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Accidental, additional, appearing casually.
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Not congenital; acquired.
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Developing in an unusual place or from an unusual source.
Synonyms:
accidental, spontaneous, sporadic, extrinsic, acquired