Chemistry
Etymology
From "chemistry" the science, via Medieval Latin alchimia from Arabic al-kīmiyā. The interpersonal sense is a 20th-century metaphor borrowing the image of two substances that react on contact — flag this as a figurative extension, not an old meaning.
noun · /KEM-uh-stree/
The involuntary sense of spark or ease between two people that neither can fully manufacture.
What it names
The part of connection that resists effort — the reaction that is either there or is not, no matter how much you both want it.
Example
The date was pleasant, the conversation flowed, and yet there was simply no chemistry.
Related idea
Distinct from rapport, which you can build through attention and skill. Chemistry is diagnostic rather than constructive — it reports a fact about two people together, and unlike affinity it need not rest on anything shared or admired.