More traditionally, appositions were called by their Latin name appositio, although the English form is now more commonly used. It is derived from Latin: ad (“near”) and positio (“placement”).
Apposition often results when the verbs (particularly verbs of being) in supporting clauses are eliminated to produce shorter descriptive phrases. This makes them often function as hyperbatons, or figures of disorder, because they can disrupt the flow of a sentence. For example in the phrase: "My wife, a nurse by training,...," it is necessary to pause before the parenthetical modification "a nurse by training."
Apposition is also a medical term used for describing when two tissues are surgically put together. For example, when two raw edges of a tissue are oversewn and sutured together to create serosa-to-serosa apposition. Apposition is the holding together of the edges of two tissue structures.
In a non-restrictive appositive, the second element parenthetically modifies the first without changing its scope. Non-restrictive appositives are not crucial to the meaning of the sentence. In a restrictive appositive, the second element limits or clarifies the foregoing one in some crucial way. For example in the phrase "my friend Alice," Alice specifies to which friend the speaker is referring and is therefore restrictive. On the other hand, in the above example: "my wife, a nurse by training,...," the parenthetical "a nurse by training" does not narrow down the subject, but rather provides additional information about the first element, namely, "my wife." While a non-restrictive appositive must be preceded or set off by commas, a restrictive appositive is not set off by commas.
Not all restrictive clauses are appositives. For example, Alice in "Bill's friend Alice ..." is an appositive noun; Alice in "Bill's friend, whose name is Alice, ..." is not an appositive but, rather, the predicate of a restrictive clause. The main difference between the two is that the second explicitly states what an apposition would omit: that the friend in question is named Alice.
The same words can change from restrictive to non-restrictive (or vice versa) depending on the speaker and context. Consider the phrase "my brother Nathan." If the speaker has more than one brother, the name Nathan is restrictive as it clarifies which brother. However, if the speaker has only one brother, then the brother's name is parenthetical and the correct way to write it is: "my brother, Nathan,...."
In several languages, the same syntax which is used to express such relations as possession can also be used appositively. Examples include:
Randolph Quirk, Sidney Greenbaum, Geoffrey Leech, Jan Svartvik A Comprehensive Grammar of the English Language London and New York: Longman, 1985 ISBN 0-582-51734-6