偶然
ǒu rán
by chance, accidentally, coincidentally
adverb/adjective
Example
我偶然在书店遇到了老朋友。
Wǒ ǒurán zài shūdiàn yùdào le lǎo péngyǒu.
I ran into an old friend by chance at the bookstore.
This word expresses the concept of something happening unexpectedly or without planning. It's commonly used in both spoken and written Chinese to describe coincidental events.