Frustration
Frustration is when the target of an aspect leaves before the aspect can complete. One significator is closing on the other, and the other perfects an aspect to a third planet first, so the matter is taken out of reach.
The fuller picture
Coley's definition is the clear one: a light planet applies to the aspect of a heavier one, and before it arrives the heavier planet meets the body or aspect of some other planet. Put in terms of a chart, one significator is applying to the other, and the other is applying onward to a third planet and gets there first. The approaching planet was going to complete something and doesn't.
One sentence separates this from prohibition, and it is worth memorising because everything else about the two looks alike. In frustration the target leaves. In prohibition either the applicant itself gets diverted onto something else, or an outsider overtakes it to reach the target first. Both configurations present as two applying aspects that share a planet with the faster one perfecting first, so the geometry won't tell you which you're looking at — you have to ask which planet moved on.
The illustration in the source is a clean one. Mars at 10° Aries is applying to the square of Jupiter at 15° Cancer, five degrees from perfection. Jupiter is meanwhile applying to the trine of Saturn at 16° Scorpio with a single degree left, and completes that first. Mars is frustrated. The figure that shows prohibition sits on the same house cusps with different planets in them, which is a deliberate pairing — the two failures are meant to be compared side by side.
What frustration gives you that a plain no doesn't is the reason. Read the house ruled by the third planet, the one the target went to, and you have why the matter was blocked. The worked case is a marriage question: the ruler of the first is applying to the trine of the ruler of the seventh, and the ruler of the seventh conjoins the ruler of the third first. The other party fell in love with the querent's brother.
Notice how much that reading depends on getting the mode right. In a prohibition the house you read belongs to the planet that came in from outside — the interloper. In a frustration it belongs to the planet the target went off to. Different houses, different people, different account of what happened, and the same three planets in both.
A last practical note on how hard a stop this is. The traditional geometry identifies what happened, but the chart must still show that the third planet can actually damage this matter. A destructive and sufficiently powerful intervention cuts the route off; a helpful or ambiguous one is not turned into a no merely because its contact perfects first.
In your chart
iHorary reserves frustration for the strict traditional bodily-conjunction pattern: a swifter planet approaches a heavier one, but the heavier planet conjoins a third planet first. The old same-target race is retained as rival pre-emption, not relabelled frustration. As with other interpositions, the geometry is recorded separately from its judged role; it becomes a denial only when the chart supports a destructive, sufficiently powerful intervention.
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