Coaxial cable is an electrical cable consisting of a round conducting wire, surrounded by an insulating spacer, surrounded by a cylindrical conducting sheath, usually surrounded by a final insulating layer. It is used as a high-frequency transmission line to carry a high-frequency or broadband signal.

Coaxial cables may be rigid or flexible. Rigid types have a solid sheath, while flexible types have a braided sheath, both usually of thin copper wire. The inner insulator, also called the dielectric, has a significant effect on the cable's properties, such as its characteristic impedance and its attenuation. The dielectric may be solid or perforated with air spaces. Connections to the ends of coaxial cables are usually made with RF connectors.

The most common RG designations seen these days are RG-6, RG-8, RG-11, RG-58 and RG-59. RG-58 and RG-8 are 50 ohm coaxes, used in radio transmission (e.g., CB or Amateur radio) or in computer networks, RG-8 being a rather large cable and RG-58 a smaller cable. As 50 ohm cables, these are unsuited for video work. RG-6, RG-59 and RG-11 are all 75 ohm cable types, with RG-59 being the smallest, RG-6 in between, and RG-11 being the largest. RG-11 is practically absent from home a/v applications, because it is very large, not very flexible, and completely incompatible with RCA connectors; its main uses are in very long runs where low signal loss is of paramount importance. RG-6 and RG-59 are both common in home a/v use, because their sizes are compatible with a variety of connectors. Both are available in many different types, with different shields, jackets, dielectrics, and center conductor materials.