An appositive is a word or phrase that renames an earlier element in a sentence. Two elements that have this relationship are said to be in apposition. For example, in the following sentence, “the T.S.A. spokesman” is an appositive of “Mr. Fotenos” because the latter renames the former:
Mr. Fotenos, the T.S.A. spokesman, promised improvements in a few months, as the agency’s Secure Flight Program takes full effect. [NY Times]
Restrictive vs. nonrestrictive appositives
A restrictive appositive is an appositive that provides crucial clarifying information about the previously named element—for example:
[A]s my esteemed colleague David Gibson noted, polling suggests that American Buddhists are less likely to step out on their spouses than Protestants. [Politics Daily]
In this sentence, without the appositive “David Gibson,” “my esteemed colleague” would be ambiguous.
A nonrestrictive appositive adds information that is not essential to the meaning of the sentence—for example:
Dennis Yohnka, my esteemed colleague at The Daily Journal, got on his “high horse” last week. [Daily Journal]
Here, without the appositive “my esteemed colleague at The Daily Journal,” we’d have no trouble understanding whom the writer is referring to—Dennis Yohnka.
Appositives and commas
As shown the above examples, nonrestrictive appositives are typically surrounded by commas, while restrictive appositives aren’t. There are exceptions in practical usage, but you can’t go wrong following this rule.