Push notification
Also called: APNs, FCM, Device token
A message delivered to a phone by Apple's APNs or Google's FCM on behalf of your server, addressed by a device token.
Definition
The app registers with the OS and receives a device token, which it sends to your server on every launch. To notify the user, your server sends a small payload and the token to APNs or FCM. Invalid tokens (uninstalled apps) must be deleted when the provider reports them, transient errors retried with backoff, and sensitive data kept out of the payload because it appears on the lock screen.
In practice
"Your order 1042 has shipped" is sent to both of a user's devices; one token returns 410 Unregistered and is removed.