Tether (USDT) is a blockchain based stablecoin - a cryptocurrency which price is linked to an asset - backed by an equivalent amount of fiat currencies like the US dollar (USD), Euro (EUR) or Japanese Yen (JPY).
It was one of the first fiat-collaterized stablecoins, with each token originally backed by 1 US Dollar (USD) and has sinced developed to be backed by loans to affiliate companies as well.