Appositive with Commas
A word, phrase, or clause in apposition to a noun is usually set off by commas (dashes or parentheses might also be used):
The chairman of the committee, Senator Jones, rambled on and on.
The press surrounded the witness—the chief accuser of the candidate—as she exited the capital building.
Her husband, David, accompanied her to the hearings.
Appositive Without Commas
Here's the exception: If the appositive serves to identify the noun, then omit the comma:
His brother Tom came to dinner.
The committee defined the term specifically by referring to the dictionary.
In the first example above, the restrictive appositive Tom, without commas, means that he has more than one brother. If he has just one brother, then the appositive is nonrestrictive and requires commas:
His brother, Tom, came to dinner.