Universal Naming Convention
Uniform Naming Convention
where "servername" is the hostname or IP address of a network file server, and "sharename" is the name of a shared directory on the server. This is related to the conventional MS-DOS "C:\windows" style of directory name. E.g.
on a server called "server1".
Even Microsoft don't know whether UNC stands for "Universal Naming Convention" or "Uniform Naming Convention", both appear on their website, sometimes withing the same document, but with a preference for "Universal".