Ghosting is a technique used by the admin of a server where, instead of being in-world, they watch the console window of the server software which supplies chat logs and allows them to speak through the console to the users in-world. This allows moderation of a server without having to use the Minecraft client to monitor the world and therefore waste resources.
To speak into the server, the admin uses the 'say' command in the console window. Their message appears in the server just as if an op had used the command, in purple text prefixed with “[Server]”. These omnipresent messages may cause player confusion but can be effective in defusing hostile situations such as arguments, verbal abuse and threats of griefing. From this window, the admin can perform all "admin commands" such as kicking and banning.
The name comes from the fact that the admin has no physical manifestation in the server, and is a disembodied voice.