Movie "chemistry" -- what is it exactly?

The idea for this thread was prompted by the “wooden acting” thread. But, often when reading reviews and film criticisms, I hear that word “chemistry.” Or the lack of it , more often than not. “The [stars-of-the-movie] just don’t have good chemistry together.” All those cheapie "E! Entertainment specials about "making of [insert-name-of-1980s-blockbuster-here] " always begin with producers talking their search to cast actors with the ‘right chemistry’. so just what IS movie chemistry?

If characters have “chemistry” it means you really care whether or not their relationship works. Chemistry doesn’t have to be entirely romantic. Misery worked because the wacko connection between James Caan (a writer) and Kathy Bates (his deranged leg-breaking fan) keeps you interested until the very end.

If chemistry is lacking, the audience will be indifferent. Needless to say, indifference makes for lousy box-office. “Chemistry”, being a hard-to-define concept, is an easy scapegoat for a director/producer to use when his latest effort bombs.

Sometimes you get negative chemistry and the audience can’t believe these two characters would be together at all, despite all the contrived plot twists (i.e. stalled elevators) that conspire to prove they’re meant for each other. This pretty much describes every Madonna movie.