What is "middlebox"

n. (context computing networking Internet English) A device in the Internet that provides transport policy enforcement, such as a firewall.

A middlebox or network appliance is a computer networking device that transforms, inspects, filters, or otherwise manipulates traffic for purposes other than packet forwarding. Common examples of middleboxes include firewalls, which filter unwanted or malicious traffic, and network address translators, which modify packets' source and destination addresses. Dedicated middlebox hardware is widely deployed in enterprise networks to improve network security and performance, however, even home network routers often have integrated firewall, NAT, or other middlebox functionality. The widespread deployment of middleboxes and other network appliances has resulted in some challenges and criticism due to poor interaction with higher layer protocols.

Lixia Zhang, the Jonathan B. Postel Professor of Computer Science at the University of California, Los Angeles, coined the term "middlebox" in 1999.