A system that monitors for malicious or unwanted activity, and either raises alerts when such activity is detected or blocks the traffic from passing to the target.
CyberWire Glossary link: https://thecyberwire.com/glossary/intrusion-detection-system
Audio reference link: “Network Intrusion Detection and Prevention - CompTIA Security+ SY0-501 - 2.1,” Professor Messer, uploaded 16 November, 2017
