> Nostr Message → Uhm, wait what? What are we talking about.

SMS (Short Message System), DM (Direct Message), Messanger, etc

A message, what else could be in a social context? A communication between two parties.

This seems quite logical and straightforward.

> Nostr DMs → Oops, that cannot be a thing without a centralized server.

Why? NIP-17 are decentralized DMs. And the NIP actually call them exactly "direct messages".