An Atomic Swap refers to a peer-to-peer transaction of a cryptocurrency between different blockchains, without the need for a crypto exchange. Tier Nolan is still considered the inventor of this approach.
Ultimately, Atomic Swaps are a type of exchange agreement through which two parties can trade tokens from two different blockchains. This primarily makes crypto users independent of centralized platforms such as exchanges. The way the exchange takes place is regulated in a form of Smart Contract.