Plain English Meaning

Near the magnetic poles, a magnetic compass stops giving trustworthy headings because the magnetic field is pulling almost straight down instead of pointing sideways. In those areas, pilots use true north for navigation instead of magnetic north.

Full Definition

A condition in high-latitude regions where the Earth's magnetic field lines converge so steeply toward the magnetic poles that magnetic compasses and heading systems referenced to magnetic north become inaccurate or unusable. In areas designated as regions of magnetic unreliability, navigation must be conducted using true north references rather than magnetic north, and charts in these areas show courses and bearings in degrees true.