A Decentralized Application (DApp) is a Software Application with a backend that is supported by a decentralized peer-to-peer, open source network such as Ethereum's Blockchain.
A typically DApp has no single entity that controls the network and does not rely on a central server to authorize, validate and serve clients. Instead DApps rely on the network and all nodes coming to consensus on valid or invalid changes to the data present in the application.
Narayan Prusty, Building Blockchain Applications. 2017, Packt Publishing