Also known as "value triple" or sometimes (not entirely correctly) "delta triple" or simply "triple". The triple is a collection of three value sets: minus set, zero set and plus set:
Minus set is a collection of values that are to be removed. This is often taken from the delete part of modification delta.
Zero set is collection of values that are not changed. These values are not added nor deleted by a specific delta.
Plus set is a collection of values that are added. These are new values that were not previously in the property, but these may also be values that are added to the second time.
Triples are usually intermediate results of computation with the deltas. It is not easy to directly merge a lot of deltas, especially if some of them are transformed by mappings, originate in various templates and roles and so on. Therefore midPoint works with triples instead of deltas. Triples are much easier to merge and they are generally better to work with.