A duplex is a house divided into two separate homes, one on top of the other or side by side. Each home has its own entrance, kitchen, bathroom, and living spaces, but they share a common wall or floor. Think of it like an apartment building, but with only two units instead of many.

In a side-by-side duplex, two families live next to each other, sharing a wall down the middle. In an up-and-down duplex, one family lives on the first floor while another lives upstairs. Each family has complete privacy within their own space, but they're connected by the building's structure.

People sometimes buy duplexes as investments: they might live in one unit and rent out the other to help pay the mortgage. Others rent both units to tenants. Duplexes offer a middle ground between owning a single-family house and managing a large apartment building.

You might also hear duplex used in printing to describe printing on both sides of a page, or in telecommunications to describe two-way communication. But when people talk about a duplex without any other context, they usually mean the two-unit house.