A terahertz is one million million (or 1012) hertz, a measure of frequency. At one terahertz, each cycle is one picosecond, and the wavelength in free space is roughly 300μm.
Radio waves sent at terahertz frequencies usually travel in line of sight. These waves, known as terahertz radiation, are in a waveband that is the overlap of what is normally regarded as microwave radiation and far-infrared light. The Earth's atmosphere is a strong absorber of terahertz radiation, so the range of terahertz radiation is quite short. However, recent technologies using terahertz radiation have been developed, which are intended for applications such as medical imaging and surveillance.