Aspect
An angle between two planets, measured in degrees around the zodiac circle. A handful of specific angles put the pair in conversation, and the closer to exact, the stronger the tie.
Positions in astrology are measured along the ecliptic, so any two bodies stand some angle apart, from zero to 180 degrees. When that angle lands on one of a few privileged values, the two are in aspect: they behave as a pair rather than as strangers.
The five major aspects, codified by Ptolemy in the second century, divide the circle by small whole numbers: conjunction at zero degrees, sextile at sixty, square at ninety, trine at 120, opposition at 180. Each has its own character, running roughly from fusion through friction to standoff. Minor aspects exist, but the majors carry most of the weight.
Aspects are never razor edges. Each is allowed an orb, a margin around exactness, and the tighter the orb, the louder the aspect. Whatever else astrologers argue about, they agree on that.
The same geometry gets read in three settings: within one natal chart, between the moving sky and a natal chart, which is a transit, and between two people's charts, which is synastry.