1. What does it mean?
케미 shortens 케미스트리, from English 'chemistry.' It describes how well people connect or how entertaining their interaction feels, including friends, colleagues, and performers.
Also written or said as 케미스트리, chemistry
2. The nuance
It does not have to imply romance. A comedy duo can have excellent 케미 without playing a couple, and an appealing on-screen dynamic is not evidence of a private relationship. 케미가 좋다 is the straightforward way to say the dynamic works.
3. Examples
“두 진행자는 성격이 다른데 방송에서는 케미가 좋아.”
The two hosts have different personalities, but their on-air chemistry is great.
“같이 일해 보니 우리 팀 케미가 생각보다 좋네.”
Now that we've worked together, our team clicks better than I expected.
Sources
KoreanClass101: Korean language lessons
References support the meaning and usage. Example sentences are written for NoCapWiki.