A joint is a smokeable marijuana cigarette, with cannabis flower rolled in a thin rolling paper. A joint can also be called a “doobie†or a “doob†mostly from the past generation but means the same thing. One can also roll a joint with a filter, sometimes called a crutch that can be purchased at smoke shops or dispensaries to improve air flow, reduce waste (the last little bit of a joint, commonly called a “roach†which some people save for a rainy day). Pre-rolled joints from a dispensary usually come with a filter already on it for your smoking pleasure. Other terms include a pinner, which means a thin joint, or a fatty which is a thicker joint.
Related terms would be a spliff, which is similar but includes a mix of cannabis and tobacco, or a blunt which is rolled cannabis in a hollowed out cigar paper.