The Ascendant is the sign rising on the eastern horizon at your birth. It is the mask that fits so well it becomes real — and your body's default posture.
The Midheaven (Medium Coeli) is the highest point in the sky at your birth. It marks the calling — the public role you were born to grow into.
The Descendant sits directly opposite the Ascendant — the western horizon. It describes the significant other and what you project onto them.
The Imum Coeli is the lowest point of the sky at your birth — directly beneath your feet. It is the private root, home, and family of origin.