Rising sign
The sign coming up over the eastern horizon when you were born. It shapes first impressions.
As the Earth turns, a new sign rises over the eastern horizon roughly every two hours. The one rising at your birth is your rising sign, also called the Ascendant.
It's the lens everything else in your chart is seen through: your manner, your style, how strangers read you. It also sets where your houses begin, which is why an accurate birth time matters.
Don't know your birth time? Your Sun and most planets stay the same; only your rising sign and houses become uncertain.