If a cable is long relative to the wavelength of the signal it is conveying it is said to behave as a 'transmission line' and the signal is passed as an eletromagnetic wave along the cable. In this condition, the cable itself has a 'characteristic impedance' which can be thought of as the impedance measured at the input if the cable is infinitely long, and it depends on the cable construction — the materials and their geometries — rather than the length. It is necesary to terminate each end of a transmission line with the same impedance as its characteristic impedance. if this is not done the propagating wave will be reflected from the ends of the cable, and the reflected energy can interfere with the source signals.
The characteristic impedance of typical coaxial video and S/PDIF digital cables is 75 Ohms, while AES3 and RJ45 Ethernet cables is around 110 Ohms.