Each of us has our own idea of what chemistry in a relationship means. To some, it could be effortlessly bantering back and forth. To others, it could be the ability to know exactly what the other person is thinking and feeling. Chemistry can be instant, or it can grow over a period. It solely depends on the bond you share with your partner.
That's connection, not chemistry. You can have a brother-sister type of relationship with someone where you have this while there is no chemistry.
Chemistry is what Duttoneer implies: this strong desire to be with someone, to see someone (again), to be around them, to spend time with them. An invisible pull, like magic. Generating strong feelings in each other, feeling mesmerised by them.
Usually with a sexual vibe.
It can be only sexual, then there's no real connection and no desire to commit, just chemistry.
In a promising relationship there is all this: admiration, chemistry, connection, commitment, compatibility.