Terahertz means a trillion cycles a second. Most often, the phrase is applied to a type of radiation that has a frequency of about a trillion cycles per second. The term could also apply to anything that happens a trillion times a second, like certain atomic vibrations or futuristic computers with clock speeds several hundred times faster than today's. In technology and industry, terahertz waves are of great interest because this portion of the spectrum is one of the hardest to generate and is just beginning to be exploited. Terahertz radiation is sometimes considered a subset of infrared radiation.
The terahertz part of the electromagnetic spectrum is defined as radiation with a frequency between between 300 gigahertz (3x1011 Hz) and 3 terahertz (3x1012 Hz), corresponding to wavelengths between 1 millimeter and 100 micrometers. This puts these waves between long-wavelength infrared and short-wavelength microwave radiation. For their wavelength below a millimeter, these waves are also called submillimeter waves, as reflected in the astronomy facilities that capture these waves from the cosmos, like the Caltech Submillimeter Observatory in California and the Heinrich Hertz Submillimeter Telescope in Arizona.
Like infrared waves, which terahertz waves are sometimes considered a portion of, terahertz radiation is emitted in small amounts by all objects with any temperature, which means everything in the universe. However, unlike waves in the near infrared spectrum, terahertz waves are found in small amounts. Like infrared and microwaves, they travel in straight lines, and are non-ionizing, safe, and non-radioactive. They can travel through a variety of non-conducting materials, including clothing, paper, cardboard, wood, buildings, ceramic, and plastic. They can also travel through fog and clouds — more effectively than infrared — but not metal or water. Like infrared light, these waves are almost completely blocked by the Earth's atmosphere.
Terahertz waves have proven challenging to generate and observe, as reliable terahertz radiation sources with only developed in the 1990s. These include the gyrotron, the backward wave oscillator, synchrotron light sources, far infrared laser, quantum cascade laser, free electron laser, and photomixing sources. Since the 1990s, research into these waves has taken off, through the commercialization and application of this radiation has been slow. Applications which have been floated include medical imaging, security, materials analysis, the study of condensed matter in strong magnetic fields, submillimetre astronomy, viewing old layers on paint on a piece of art, satellite-to-satellite or aircraft-to-satellite communication, and quality control imaging for manufacturing.