|This article/section is a stub — probably a pile of half-sorted notes and is probably a first version, is not well-checked, so may have incorrect bits. (Feel free to ignore, or tell me)
Appositions (not to be confused with adpositions) are used to help clarify and/or disambiguate, often adding context/explanation/definition/description/name.
In English (and presumably in many other languages), appositional modifiers are typically placed directly next to the things they augment.
Typically both parts of the relation are nominals, and often full nominals, (which also implies that often, you could create a correct sentence with either(/any) of the parts, though this can be strained, and don't count on it having/attaching the same semantic meaning; it's more about whether the reference is fairly independent)
- the parenthetical in "John, a mathematician, has a habit of mumbling"
- adds contextual information to John
- abbreviations next to their expansion
- in men and women, both are put it a more limited and specific context
- which in this particular case suggests that this context we are grouping or distinguishing gender.
- In The name John stands for ..., name is an appositional modifier for John
- You can have multiple
- Bob, your sister, my lover, ...
- since the explanation can be long, the appositional relation between a phrase's head can be some words away
- In "The leader of the national opposition, Bob, ...", leader has an appositional relation to bob,
- the previous example is also an example that, while a lot of apposition is fairly parenthentical and punctuation is a good indication, there are variants that do not look like that