Rogue Wave (extreme wave)
Wave more than ~2× the significant wave height; rare but documented and dangerous.
In practice
Reported to NOAA WAVEWATCH database and the relevant flag state. Damage typically affects bow plating, anchor stowage, and bridge windows. Standard procedure: reduce speed, head into the seas, dispatch damage report.
Regulatory detail & full definition
Once dismissed as sailor folklore, confirmed by satellite radar (Christmas wave 1995, Draupner platform) and statistical analysis. Particularly common in regions of strong opposing currents (Agulhas current off South Africa, Gulf Stream meander) and converging seas. Modern ship-routing tries to avoid known hot spots.
Related
Source: WMO/IOC JCOMM; Draupner wave incident analysis
High confidence
Editorial confidence: how we grade.