Astrological compatibility (synastry) is the branch of astrology that studies relationships by comparing natal horoscopes. A natal horoscope is a chart, map, or imaginary snapshot of the planets in the Solar System and their positions in the zodiac at the exact time of a person’s birth. The signs of the Zodiac represent 12 different types of spiritual energy and purpose. The planets in the Solar System are associated with at least one, but most often two signs of the zodiac.
Relationships between planets, signs (types of energy), and houses (sectors of the chart) are described in words and/or often rated in numbers to show to what degree, how, and in what ways one person is compatible or incompatible with another. The assessment is based on the patterns and distributions of energy in the respective and combined charts.
Definition courtesy Wikipedia.