cou·pler / ˈkəp(ə)lər/ • n. something that connects two things, esp. mechanical components or systems: a hydraulic coupler. ∎ Mus. a device in an organ for connecting two manuals so that they both sound when only one is played. ∎ Mus. (also octave coupler) a similar device for connecting notes with their octaves above or below. ∎ Photog. a compound in a developer or an emulsion that combines with the products of development to form an insoluble dye, part of the image. ∎ (also acoustic coupler) a modem that converts digital signals from a computer into audible sound signals and vice versa, so that the former can be transmitted and received over telephone lines.