An Atomic Swap is a smart contract technology that allows the exchange of one cryptocurrency for another without the use of centralized intermediaries, such as exchanges. Atomic swaps can take place directly between blockchain of different cryptocurrencies, or they can be done off-chain, away from the main blockchain.
They first appeared in September 2017, when an atomic exchange between Decred and Litecoin took place. Since then, Lightning Labs, a startup that uses bitcoin's Lightning network for its transactions, has made off-chain exchanges using the technology.
Decentralized exchanges, based on Smart Contracts, such as dXdY and Uniswap, have incorporated swap in the same blockchain.