This term has a dual meaning in the cannabis community. The smokable kind of joint is usually consisting purely of cannabis kept together with a rolling paper. They can be also be rolled up with a paper filter (also referred to as a crutch or roach).
Another definition of a joint is the connection allowing bowls, bangers, ash catchers, among other attachments, to connect to a water pipe. Joints are sized according to their diameter, with industry standards being 10, 14, and 19mm, with 14mm being the most common size of joint. Having standardized joints allows for a plethora of attachments to be used with your pieces.