A place. Can be any place, doesn't have to be jail or a crack house.
"The joint", however, means the fucking slammer.
Delonte: "Ayyo nigga let's hit up dat science museum tomorrow."
Jameer: "Hell yeah dogg that joint is off da heezyfo' sheezy."
Delonte: "Yeah that joint sho' is better than THE joint."
In NYC in the 70's, the joint meant that something was very good or the best. It could be just about anything.
Ex. 1. Yo, did you see that movie last night?
Yeah, it was the joint.
2. I heard Parliament last night. Tear the Roof of the Sucker is definitely the joint.
3. This pizza is the joint!!~