I know this has probably been addressed before, but it's driving me crazy. What are "rubbers" (as in "Lady Middleton, though in the middle of a rubber, on being informed that Marianne was unwell, was too polite to object for a moment to her wish of going away...")?

I know it has something to do with the card game Casino, but I'm unfamiliar with it so I have no idea what is being referenced. Could someone point me to a previous message or a web page or something? I've noticed the word in this contex in several JA books and it's bugging me!