Glossary
Meridian
The local meridian is the great circle running from due north to due south through the point directly overhead. Where it crosses the ecliptic defines the Midheaven above and the Imum Coeli below — the meridian axis of a chart.
Meridian lines (MC/IC) depend only on the rotation of the earth, not the local horizon, which makes them far more forgiving of an approximate birth time than rising and setting lines. When a chart says 'these lines survive a fuzzy birth time,' it means the meridian ones.