An extremely high frequency 5G cellular band. Millimeter Wave (mmWave) 5G cellular bands are broadcast at 24GHz or higher, and are capable of insanely fast speeds. But mmWave signals have extremely limited range, and can be easily blocked by walls and even windows.
AT&T has chosen to dub its mmWave service as 5G+, and Verizon calls it "5G Ultra Wideband" or "5G UW".