Coaxial cable, often referred to simply as coax, is a common type of cable used in audio and video work. It consists of a central conducting wire surrounded by an insulating layer (dielectric), which in turn is surrounded by a conducting sheath and a further insulating layer.
The central wire conductor carries the signal, the sheath acts as a shield. By restricting electromagnetic waves to the area between the centre conductor and the shield, coaxial cable protects the signal from unwanted interference caused by external electromagnetic fields.
Most coaxial cables have one of the following characteristic impedance ratings: 50, 52, 75, or 93 Ω. 75 Ω is the most common rating for professional video cables.