Next door means in the building, room, or house immediately beside yours. If your best friend lives next door, they're in the house right beside yours, so close you could probably see into their yard or hear them playing outside. In an apartment building, a next-door neighbor might share a wall with you.

The phrase captures a special kind of nearness: your next-door neighbors are the closest people to you geographically. This closeness often matters: you might borrow sugar from the family next door, notice when their dog is barking, or wave to them every morning.

People also use the phrase to mean “very close” in other situations. A student might say the library is next door to the cafeteria, meaning they're right beside each other. Or you might describe someone as “the next-door type,” meaning they seem familiar and approachable, like a friendly neighbor.

The hyphen connects the two words when they come before a noun (your next-door neighbor) but usually disappears otherwise (she lives next door).