Cisco Network Admission Control
Abbreviated as NAC, Cisco's Network Admission Control is a set of technologies and solutions that use the network infrastructure to enforce security policy compliance on all devices seeking to access network computing resources in order to limit damage from emerging security threats. NAC is designed to allow network access only to compliant and trusted endpoint devices (PCs, servers and PDAs, for example) and can restrict the access of noncompliant devices. NAC assesses all endpoints across all access methods, including LAN, wireless connectivity, remote access and WAN. [Source: Cisco Systems]
In October 2006, Cisco and Microsoft formally announced interoperability between the Cisco Network Admission Control (NAC) and Microsoft Network Access Protection (NAP) solutions. Interoperability will be supported with the release of NAP in the future version of Windows Server which is scheduled to be available in the second half of 2007. The interoperability architecture allows customers to deploy both NAC and NAP incrementally or concurrently.
See also Microsoft Network Access Protection (NAP).
Also see Webopedia's "Did You Know... All About Network Access Controls".