Terahertz (THz) is a unit of measure of frequency that is equal to 1 trillion hertz (1012 Hz). It usually refers to the frequency of an electromagnetic wave, which is a part of the electromagnetic spectrum invisible to the naked eye that lies in between the microwave and the infrared range. T-rays are an example of a specific part of the spectrum within the ITU-designated band and range from 0.3 to 3 THz, which finds utility in astronomy.
Techopedia explains Terahertz (THz)
The terahertz frequency radiation (also known as submillimeter radiation, which has a wavelength of 0.1 mm or 1 µm up to 1 mm) occupies the middle ground between the lower microwave spectrum and the infrared spectrum known as the terahertz gap. It is called a gap because, compared to the microwave and infrared spectrum that are used extensively, the technology for the creation and manipulation of radiation in the terahertz frequency is in its infancy, and there are only few implementations. This is because, at these frequencies, the electromagnetic radiation becomes too high or too thin to be digitally measured using electronic counters, and so it must be measured in proxy through the use of the properties of wavelength and energy. The generation and modulation of electromagnetic signals in this range are also very hard to do with conventional electronic devices used in generating radio waves and microwaves, which would require further research into new technology.
Terahertz radiation has a hard time penetrating fog and clouds and cannot penetrate liquid water or metal, which makes its use quite limited to indoor environments. It is also non-ionizing, which means it does little to no damage to living tissue. This also means that there is no immediate need to make use of the frequency for a wider audience, such as in telecommunication, because of its drawbacks, but it may find many uses in other fields such as medical imaging because of its non-ionizing properties.