Decentralised exchange (DEX)
An exchange run by smart contracts, where you trade from your own wallet.
In plain English
You keep custody until the moment of the trade, which removes one risk and adds others: smart-contract bugs, fake tokens, and the fact that there is no support desk if you make a mistake.
Example
Swapping tokens on a DEX sends them from your wallet to a contract and back, with no account to open.