A cryptocurrency which is deposited with a collateral and therefore should have less volatile properties than conventional cryptocurrencies. They aspire towards a stable price.
Many stablecoins are linked 1:1 with certain fiat currencies, such as the US dollar or the euro. They can be traded on cryptoexchanges, often as a currency pair against the largest cryptocurrencies.
Stablecoins can also be linked to other types of assets, such as precious metals or other cryptocurrencies.
There are also projects that use a basket of assets as a price base. Facebooks Libra should have these features.
Centralized Stablecoins show a low volatility, but have an additional counterparty risk compared to pure cryptocurrencies.