Definition
An event kind is the integer field, defined in NIP-01, that every Nostr event carries to declare what it is. Kinds tell clients how to interpret an event's content and other fields, and just as importantly, the numeric range of a kind tells relays how to store it. Understanding kinds is the key to reading any Nostr feature, because every NIP ultimately assigns its messages a kind.
Special and regular kinds
Kind 0 is user metadata, where the content is a JSON profile describing the author. Kind 1 is a plain text note, the basic social-media post. Most other low and mid-range numbers (1-44 and 1000-9999) are "regular" events that relays store normally without replacing older ones.
How ranges govern storage
The high ranges encode behavior. Replaceable events (kinds 0, 3, and 10000-19999) keep only the latest event per pubkey-and-kind pair; older versions may be discarded. Ephemeral events (20000-29999) are not expected to be stored at all, suiting real-time signals. Addressable, or parameterized-replaceable, events (30000-39999) are stored uniquely per combination of kind, pubkey, and d tag, which is how things like long-form articles and marketplace listings stay editable in place.
Because the range dictates persistence, you can often predict an event's lifecycle just from its number. This underpins everything from relay routing to commerce; see the addressable kinds used by the Nostr Marketplace (NIP-15) and the request ranges of the Data Vending Machine (DVM).
Full open-data reference: Nostr Event Kinds Reference — CSV / JSON + REST API, CC BY 4.0.
In Simple Terms
An event kind is the integer field, defined in NIP-01, that every Nostr event carries to declare what it is. Kinds tell clients how to…