In cryptocurrency a DEX is a Decentralized peer-to-peer Exchange. It is an exchange run by code. It usually exists as a decentralized application (DApp). It is a place where people can trade cryptocurrencies directly without a middleman.
On a DEX every transaction is generally written to the blockchain. The DEX replaces the need for a centralized exchange to act as a middleman.
One example of a DEX is EtherDelta. EtherDelta is a token exchange that exists as a DApp and utilizes the Ethereum blockchain.
NOTE: Although generally a DEX will have no middleman when it comes to trades (all trades are between two peers), there are generally developers who manage the platform and aspects of a DEX thus won’t be fully decentralized. In general a DApp needs a front end, and that front end is often not coded onto a blockchain, and thus must be managed to some degree. Likewise, a team of developers may end up working on the code, and who works on the code may be controlled by a team. Also, developers may be able to do things like add and remove coins for trade. In sum, how much outside control there is can differ by exchange, but since the code had to be created by someone and since aspects of it have to be managed, in general a DEX is not fully decentralized, just specifically the trading is.
TIP: Since transactions on a DEX are recorded on the blockchain, 1. they can be slow to process if transactions are congested and 2. they have no way to reverse transactions. So if you make a bad trade, even by accident, there is no way to undo it in general with a DEX.
"What is a DEX?" contains information about the following Cryptocurrencies: