Nostr: What is a Nostr event?

Short answer

An event is the basic Nostr data object. It includes a creator public key, timestamp, kind number, tags, content, event ID, and signature. The kind tells clients how to interpret it, such as a profile, follow list, or short note. A valid signature proves the event was signed by the matching private key; it does not prove the author's real-world identity.

Sources

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