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 the least sensitive lines to an approximate birth time. Steadier is not the same as free, though: like every line they need some time of day to be drawn at all, so a chart with no birth time shows no meridian lines either.