The easiest way to define web3 requires me to contrast it with web2.
Web2 is the web we know: mediated by large tech conglomerates who actually build and host the metal, electronics and code that let us become an "account" row in their own databases. This web is possible because sophisticated coporate entities let us play on their data centers.
Web3 aspires to build a different web: one where creators own their creations, community builders own their communities, and participants can voluntarily enter and exit digital realms without permissions and without given up all the history and experiences they have accumulated.
Others may mention blockchain, cryptocurrency and p2p network, but I won't 😉.