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.
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"