5G mmWave
Millimeter-wave 5G — the gigabit-fast version that only works within a city block of a tower. Mostly Verizon, some AT&T. Mostly at airports, stadiums, and dense downtowns.
5G mmWave (millimeter wave) is the highest-frequency, highest-speed flavor of 5G. Spectrum bands like n260 (39 GHz) and n261 (28 GHz). Speeds can hit 1–4 Gbps in real-world tests — but only at very short range.
The trade-off
mmWave is fast but doesn't go far. Practical coverage radius from a single mmWave tower is 100–300 meters, and the signal is blocked by walls, glass, and even rain. To deploy mmWave widely you need many tiny base stations on light poles. Carriers have done this in airports, sports venues, and downtown cores of major cities.
Where mmWave is deployed
Verizon was the most aggressive mmWave deployer (their "Ultra Wideband 5G" was originally mmWave-only). AT&T deployed mmWave at major airports and stadiums. T-Mobile deployed less mmWave; their strategy bet on mid-band being more useful in practice.
Realistically, most users encounter mmWave in airports, NFL/NBA stadium concourses, and a few square blocks of downtown Manhattan / SF / Chicago / DC. Outside those zones, mmWave-capable phones fall back to mid-band or low-band 5G.