Supersonic means faster than the speed of sound in a given medium, usually air. In engineering, it marks a regime where airflow, drag, vibration, and shock waves behave very differently from normal-speed motion. Small changes in shape, timing, or load can have large effects once a system crosses that threshold.
In cyber security, supersonic is usually used as a metaphor for systems pushed beyond their expected operating envelope. A service, network, or control process may seem stable at ordinary load, but under extreme traffic, malformed inputs, or rapid state changes it can become unpredictable. Attackers often exploit these limits with floods, race conditions, or resource exhaustion, while defenders test them with stress testing, rate limiting, and input validation. The lesson is simple: once a system leaves its design assumptions, tiny errors can become major failures.