Where two significators are applying towards conjunction, but before it perfects the slower planet has already caught up with and perfected a conjunction with another planet. This third planet can interfere with the promise of the contact between the two significators.
Lilly writes of frustration (CA., p.111)
Frustration is, when a swift Planet would corporally join with a more ponderous, but before they can come to a Conjunction, the more weighty Planet is joined to another, and so the Conjunction of the first is frustrated, as Mercury in ten degrees of Aries, Mars twelve, Jupiter in thirteen of Aries; here Mercury strives to come to Conjunction with Mars, but Mars first gets to conjunction with Jupiter; whereby Mercury is frustrated of the Conjunction of Mars: in Questions is signifies as much as our common Proverb, The Dogs quarrel, a third gets the Bone.
This is a form of abscission.