Several decentralised exchanges (DEXs) exist, but the definition of a DEX is not clear

Examples:

Barterdex: https://komodoplatform.com/decentralized-exchange/

Bit Square (bisq): https://bisq.network/

etc

Further reading on atomic swaps etc:

Vitalin Buterik: https://static1.squarespace.com/static/55f73743e4b051cfcc0b02cf/t/5886800ecd0f68de303349b1/1485209617040/Chain+Interoperability.pdf

Kyle Samani: https://www.coindesk.com/opportunity-interoperable-chains-chains/

Adrian Mathieu/Viacoin: https://ethereumworldnews.com/viacoin-developers-successfully-completed-atomic-swap-litecoin/