Radio frequency bands above 24 GHz.
Traditionally, cellular networks operated in frequency bands below 2.7 GHz.
In late 2018, the first 5G networks are launching, many of which use — or will use — one or more mmWave frequency bands.
See: 5G NR
Radio signals using mmWave do not travel as far as those using lower frequencies, and do not penetrate solids (walls, trees, and windows) as well.
In the U.S., mmWave bands licensed for 5G service include 24, 28, 37, 39, and 47 GHz. Licenses in these bands will be auctioned off to wireless companies starting in late 2018.