Originally: designating a system of telegraphy enabling four messages to be sent over a wire at the same time, two in each direction (now chiefly historical). Later also: relating to or designating the transmission or reception of four signals simultaneously over one channel or with one aerial.
Of a polyploid individual: having the dominant allele of a particular gene represented four times.
1historical A quadruplex telegraph.
2North American rare A house or apartment that has four rooms.
3An apartment that has four floors connected by interior staircases.
4A building divided into four separate residences; = "quadplex".
Early 17th century; earliest use found in Thomas Palmer (?1541–1626), author. From classical Latin quadruplex fourfold, multiplied by four, four times as much from quadru- + -plex. Compare earlier quadruple, quadruplicity.
To make (a circuit, conductor, etc.) quadruplex; to multiply (a circuit's capacity) fourfold; now chiefly historical.
Late 19th century; earliest use found in The Electrician. From quadruplex.