Nostr: Is Nostr an app or a protocol?

Short answer

Nostr is a protocol, not one app. A client is the app you use, a relay is a server that accepts and serves events, and NIPs describe interoperable behavior. This distinction matters when an app shuts down: you may be able to use another client with the same keys, provided it supports the features and can find your data on relays.

Sources

NIP-01: Basic protocol, Nostr Improvement Proposals index.