Something adventitious happens by chance. It was not planned, and it does not really belong to the main story — it just turned up. It is a formal, almost scientific word, not a word for the dinner table. Doctors and biologists use it more tightly. Adventitious roots are roots that grow from an unusual place, such as a stem, not from the usual root. Adventitious sounds in the lungs are extra sounds that should not be there. In ordinary writing, if you see it at all, it usually just means by chance, not essential. Though actually, chance here is closer to extra and from outside than to lucky. Students often hear adventurous in their head because the two words look close, but adventurous is about liking risk; adventitious is about something that simply arrived.

偶然的、偶發的。生物課的 adventitious root 是不定根。