Ascendant
In plain words
The exact point of the sky that was climbing over the horizon in the east at the minute you were born. It sets where all twelve houses begin, which is why the birth time matters so much.
In the tradition's terms
The rising degree: the intersection of the ecliptic with the eastern horizon, and the cusp of the first house in most systems.
In depth
Computed from local sidereal time, latitude and the obliquity. Moves roughly a degree every four minutes of clock time, so a four-minute error moves every house cusp by a degree.