Transmission line
A transmission line is a pair of conductors built to carry a high-frequency signal from one place to another while keeping the energy guided and contained — think of a coaxial cable feeding your TV, or the twisted pair behind your wall socket. At low frequencies a wire is 'just a wire', but once the signal's wavelength shrinks to the size of the cable, the voltage and current are no longer the same everywhere along it: a wave actually travels down the line at a fraction of the speed of light.
The trick is that a transmission line is described not by a single resistance but by distributed inductance and capacitance per unit length. These two quantities set how fast the wave travels and what 'resistance to the wave' (the characteristic impedance) the line presents. The rule of thumb engineers live by: if a cable is longer than about one-tenth of a wavelength, treat it as a transmission line — reflections, delay and impedance matching suddenly matter.
The same physics applies to a PCB trace, an undersea cable, and a power line — what changes is the frequency at which the 'just a wire' approximation breaks down. A 1 GHz signal sees even a few-centimetre trace as a transmission line.