A planet's movement in the heavens (subsequent to birth) over or in aspect to factors in the natal or progressed chart, or in aspect to other transiting factors. The closer the two bodies are in declination, the greater will be the effect of the transit; conversely, the further apart they are in declination, the lesser the effect. Transits have noticeable effects on the world, in particular on individuals' lives, according to where they occur in the natal chart. For an individual, transits are particularly effective if they coincide with progressions, not so effective otherwise.

Transit means movement through or across, so a planet may also be said to transit a sign, or a house.

Astronomically, it can also mean crossing the face of the Sun, as in the rare "Transit of Venus", or indeed any other celestial.

Classical astrologers, such as Valens, consider that a transit is only important in terms of its effect on the native's life if the planet is a time lord at the time, though I'd say this is certainly not the case for the slow-moving outer planets discovered in modern times, particularly Uranus and Pluto. I also have my doubts about the classical planets in this regard, especially Jupiter and Saturn.