The house or apartment that is the closest to yours is next door. If your best friend lives next door, you can open your windows and call to each other, instead of using the telephone.
Unless you live in a rural area where houses are miles apart from each other, you have a next door neighbor or two — people who live in the adjacent house or apartment. In a row house, town house, or apartment, the person next door even shares a wall with you. This term dates from the fifteenth century, from the sense of "nearest house" or "nearest door."