I really like this quote from Jenna Fisher (Pam from The Office) where she says, "for me, chemistry is when you have to work the least to be the most authentic as your character."

I don't think that's how most people would define chemistry if asked, but I think it's actually a really good description of what acting chemistry actually is, in the broadest and most relevant sense. It's that thing where two characters really 'click' as characters, and feel authentic, and sell it, when they play off of each other.

It's not really about romance or attraction between them; it's about how their compatibility as actors makes the fiction that's being created feel more compelling and convincing for the audience.