Tether (USDT) is a cryptocurrency that uses the Bitcoin and Ethereum blockchain to conduct financial transactions and is linked to fiat currencies, in particular the US dollar, euro and Japanese yen. For each currency, there is a token identical to its index, but with the suffix "T": USDT, EURT, JPYT.
The token was issued in 2015 by Tether Limited. The main idea of the developers of this token is to provide participants in the cryptocurrency market with the opportunity to use a stable digital asset ("stablecoin"), the rate of which is tied to the US dollar and does not experience such strong fluctuations as the rates of other cryptocurrencies. I.e., USDT in fact helps to fix the value of the crypto and insures against strong fluctuations in the financial market.