Synthetix is a protocol for issuing and trading synthetic assets on Ethereum. Each synthetic asset (or Synth) is an ERC20 token which tracks the price of an external asset; for example each sUSD token tracks the price of the US dollar (and unlike the other synthetic assets, is fixed at 1).
A wide variety of Synths exists within Synthetix, including fiat currencies, cryptocurrencies, commodities, and inverse indexes. In principle the system can support any asset with a clear price, and provides on-chain exposure to an unlimited range of real-world assets.