NOS meaning in Information Technology ?

A network operating system (NOS) is a specialized operating system for a network device such as a router, switch or firewall.

Historically operating systems with networking capabilities were described as network operating system, because they allowed personal computers (PCs) to participate in computer networks and shared file and printer access within a local area network (LAN). This description of operating systems is now largely historical, as common operating systems include a network stack to support a client–server model.