Network Attached Storage (NAS)
Network Attached Storage is a name for the technologies and protocols used for sharing files in a distributed environment. A NAS environment can be anything from a small embedded 20$ device you attach to your network and use to store files on up to huge enterprise class fileservers costing many milions of dollars.
Common for all NAS systems is that they are about sharing files and filesystems across a network. A common example might be a Linux host that serves out files using the NFS protocol.
If it is filesharing it is NAS.
The opposite to this is a Storage Area Network (SAN), which is a network dedicated to storage transfer only. In a SAN environment what you "share" is not files or filesystems but instead disks and block devices.