An aspect is an angular relationship between two points in a chart. A conjunction is centered on 0°, a square on 90°, a trine on 120°, and an opposition on 180°, for example.
Software can calculate the separation. An astrological system then decides which target angles to recognize and how far from exactness each may be; that allowed tolerance is called an orb. The aspect name therefore combines measured geometry with declared conventions.
Astrology assigns symbolic readings to aspect types, such as friction to a square or reinforcement to a trine. Those readings are reflective traditions, not scientifically demonstrated forces. A precise angle does not make its interpretation a measured fact.
Aspects can be measured within a birth chart or between a moving transit and a chart position. A reproducible page states the points, exact separation, target aspect, orb policy, coordinate convention, and timestamp.
Sources and further reading
See the project methodology for the calculation settings and the aspects guide for worked examples. The target angles, orb policy, and symbolic readings are declared conventions rather than findings attributed to an astronomy source.