To adjoin means to be next to something or to share a border with it. When two rooms adjoin each other, they touch: there might be a door connecting them, or they might just share a wall. A hotel room that adjoins yours is right next door, so close you might hear your neighbors through the wall.

The word often describes properties or spaces that touch without any gap between them. A garage that adjoins a house is attached to it, sharing a wall. Two countries adjoin when they share a border, like Canada and the United States.

You'll also see the related word adjoining: “We stayed in adjoining hotel rooms so our families could easily visit back and forth.” When something adjoins something else, they're neighbors in the most literal sense, touching at their edges. Think of adjoining as meaning “right next to” or “directly connected to,” with nothing in between.