The Midheaven is the highest point in the sky – it represents the geographic meridian (longitude) running through the location of the chart (for example, the place of birth).
The Midheaven is sometimes just called the MC, short for the Latin Medium Coeli, Latin for "Middle of the Heavens," and is usually associated with the South direction in the chart. The point opposite the MC is the IC (Imum Coeli) in the chart, and represents the North direction in the chart.
In most house systems, it is also the cusp of the 10th House in the chart. In a natal chart, the sign on the Midheaven and any planets in the 10th house, as well as the planet ruling the sign on the MC, all describe what the person aspires to, their goals, career, societal standing and reputation. The Midheaven is also associated with one of the parents, traditionally the mother.
If a chart is calculated for noon, the Sun will be located on or very close to the Midheaven in the chart. An exception is that if the chart is calculated for Daylight Saving Time, the Sun will actually be found a bit to the left of the MC in the chart, usually in the 10th House.
In the Equal House and Whole Sign house systems, the Midheaven is separate from the 10th House cusp and usually found inside the 9th or 10th house.