By now, it has become standard terminology in the research community to use the phrase "terahertz radiation" to mean EM waves between 0.1-10 THz (although some

By now, it has become standard terminology in the research community to use the phrase "terahertz radiation" to mean EM waves between 0.1-10 THz (although some people prefer 0.3-30 THz, and there are also other options out there). There is no uniform and universally agreed-upon definition. But the bottom line is this: the term is never used to refer to infrared (e.g., 100 THz) or visible light (e.g., 500 THz). This part of the spectrum has previously been lumped into the "far infrared", and sometimes called "submillimeter waves", and in the old old days was called "mega-mega cycles". The reason that there is so much confusion about the terminology to describe this spectral range is that it lies outside of the purview of both optics people and RF engineers. It is the "in between" region, which until recently was unfamiliar to most scientists. So there has been a lot of borrowing terms from different communities going on, and these communities have often not coordinated with each other. It's a bit of a mess, but we live with it.