adj. Twofold; double.adj. Having two apartments, divisions, or floors.adj. Relating to or being a single assembly of machinery having two identical units that are capable of operating simultaneously or independently.adj. Electronics Of or relating to a communications mode, as in a telephone system, that provides simultaneous transmission and reception in both directions.n. A house divided into two living units or residences, usually having separate entrances.n. Something, such as an apartment, that is duplex.