The question isnt a category error and deserves a direct answer.

If I follow what you're saying the answer could have been: "it's a framework/set of protocols for building a Twitter that can show all the stuff on other compatible Google+/Facebooks"

If you have a preference for this style of definition, then we could say that

Nostr is a protocol that's well suited for creating decentralized applications that need publicly verifiable identity, censorship resistance and event based communication.

For example, https://zapstore.dev/app is an Android AppStore that uses nostr to provide a decentralized way to verify the developers and remove "fake" apps.