The word "joint" has lots of completly different meanings. In drug-terms, a joint is a special preparing form of hashish "cigarettes" for smoke inhalation. A piece of hashish (or marihuana), mixed with tobacco, wrapped into several paper sheets. The process is called in the U.K. as "skinning up".
A synonym is "spliff", originally, a Jamaican term for a large cone-shaped smoking device carved of wood and packed full of cannabis or a cannabis/tobacco mixture. Eventually rolling papers replaced the solid cone.
Other slang terms are doobies, reefers, and zoots.