Tether is a cryptocurrency launched in 2014. It is a so-called stablecoin currently leading by market capitalization. Although according to the initial idea, each Tether was backed by $1 (that was going to keep the price of Tether stable), the company not always managed to maintain this price (at some point in October 2018 the price dropped to $0.9 per one Tether). Tether succeeds in terms of market capitalization and its price is almost all the time is really close to $1.
The price is falling again, I'm very disappointed.