A transit is a celestial body’s calculated position at a selected time, or an astrological comparison between that moving position and a reference chart. For example, a transit calculation might measure the angle from current Saturn to a Moon position in a birth chart.
Reproducible transit data requires a timestamp, time standard, coordinate and zodiac conventions, software settings, and—when houses are involved—reliable chart time and place. An aspect also needs a declared orb policy.
Astrology assigns symbolic meanings to transit combinations and durations. Those interpretations are reflective traditions, not scientifically validated event forecasts. A transit should not be used as the sole basis for medical, legal, financial, relationship, or safety decisions.
Slower apparent motion and retrograde loops can make a body cross the same chart degree more than once. A useful calendar distinguishes entry into an orb, exact contact, and departure instead of presenting a single date as guaranteed significance.
Sources and further reading
The project methodology, pinned Astronomy Engine JavaScript reference, and official NASA/JPL Horizons documentation support the position, time, and motion layers. They do not validate transit forecasting.