A Network Based Intrusion Detection System (NIDS), or Network Based IDS, is security hardware that is placed strategically to monitor critical network traffic. Traditional Network Based IDS analyzes passing network traffic and matches that traffic to a library of known attacks in its system. Newer systems use artificial intelligence (aka heuristics) to analyze traffic for patterns of interest. When an attack is identified, or a pattern of interest is found, an alert is sent typically to a Security Operations Center (aka: SOC) who review the alert and triage it for validity and subsequent escalation.
Source: Wiki Network Intrusion Detection Systems
Related Terms: Packet Sniffing, Intrusion Prevention Systems