Midheaven
In plain words
The highest point the sky reaches over your birthplace — straight up and slightly south. It is read as the public face of a life: work, reputation, what you are seen doing.
In the tradition's terms
The MC: where the ecliptic crosses the meridian, and the cusp of the tenth house in the quadrant systems.
In depth
Right ascension of the midheaven equals local sidereal time. Unlike the ascendant it does not depend on latitude, which is why it stays well-behaved at the poles when the ascendant does not.