Coaxial cable, as a common example, has a wire conductor in the center (D) a circumferential outer conductor (B) and an insulating medium called the dielectric (C) separating these two conductors. The outer conductor is usually sheathed in a protective PVC outer jacket (A).
The dimension and material of the conductors and insulation determine the cables characteristic impedance and attenuation at various frequencies.
In loudspeaker design, coaxial refers to a loudspeaker system in which the individual driver units radiate sound from the same point or axis. This is achieved by placing the high-frequency unit in the centre of the low-frequency driver, rather than having the two radiating from separate spaces. Altec Lansing pioneered the design with their 601 and 604 "duplex" (Altec's term for "coaxial") drivers in the 1940s and Tannoy further popularized the idea with their "dual-concentric" design several years later. The concept has been used by a number of other loudspeaker companies such as KEF, Technics, Thiel and Geithain, but it still remains a minority exercise due to technical and budgetary considerations despite its theoretical advantages.