The Descendant is the zodiac degree setting where the ecliptic meets the western horizon for a stated time and location. It lies directly opposite the Ascendant in the chart.
Many house systems use it as the seventh-house cusp, though house conventions should always be declared. Its calculation is time- and location-sensitive, so an unknown birth time cannot support a definite Descendant.
Astrology associates this angle with one-to-one relationships, encounter, and qualities considered through another person. That is a reflective convention, not a method for diagnosing a partner or predicting a relationship outcome.