ETC stands for exchange traded commodity. It is a type of exchange traded product (ETP), like exchange traded funds (ETFs) or exchange traded notes (ETNs).
Like other types of ETP, ETCs trade on exchanges like shares. However, ETCs seek to track the price movement of a commodity or group of commodities.
An ETC may consist of either the underlying securities it is tracking, or a swap agreement which may or may not be secured by collateral. For this reason, some ETCs can be seen as a type of ETN.