Ascendant
The zodiac degree rising on the eastern horizon at the moment of birth. It anchors the house wheel and colors how you meet the world. It moves through a full degree about every four minutes.
As the Earth turns, the whole zodiac wheels across the sky once a day, and at any moment one degree of it sits exactly on the eastern horizon, rising. The Ascendant is that degree, frozen at the instant of birth. The sign containing it is your rising sign.
Astrologers read the Ascendant as the face of the chart: temperament at first contact, the style strangers meet before they meet the rest of you. Structurally it does even more work. It opens the first house, and in most house systems the entire wheel hangs from it.
Because it comes from the Earth's daily rotation rather than from any orbit, it is the fastest-moving point in a chart, which makes it both the most personal thing there and the most fragile. A birth time off by half an hour can move the Ascendant several degrees, sometimes a whole sign. When the time is unknown, the honest move is to read the planets and let the Ascendant go.