gigahertz

The gigahertz is the microwave band, and most of the wireless traffic in a house lives in it: Wi-Fi at 2.4 and 5 GHz, Bluetooth at 2.4 GHz, mobile mid-band near 3.5 GHz, and the microwave oven at 2.45 GHz. The oven and the router share that slice by design, since it was set aside for equipment that radiates rather than communicates, which is why a worn oven door seal can stall a 2.4 GHz link across the kitchen. Radio astronomers guard 1.420 GHz, the emission line of neutral hydrogen and one of the most protected frequencies in the spectrum.

The distinctive thing about this band is that the SI second lives in it. The caesium transition that defines the second is 9,192,631,770 Hz, about 9.19 GHz, so every caesium clock is at heart a microwave receiver locked to that line. Engineering changes character up here too: at 1 GHz a cycle lasts one nanosecond and light travels only 30 cm, so a circuit-board trace a few centimetres long stops being a wire and starts being a transmission line, with geometry that behaves like part of the circuit. The wavelength solvers on this site put numbers to it, with $\lambda = c/f$ placing 2.4 GHz at 12.5 cm.