An appositive is a noun, a noun phrase, or noun clause which sits next to another noun to rename it or describe it in another way, explains the Grammar Monster. The word comes from a Latin word meaning "to put near."
An appositive is offset with commas, brackets or dashes. Terms such as "namely", "that is" or "in other words" sometimes introduce the appositive. If the appositive is merely additional information, it is a non-restrictive clause, and the sentence does not have a loss of meaning with its removal. Parentheses are an option for offsetting these clauses from the rest of the sentence. The Grammar Monster gives the following sentence as an appositive example: "Jane Smith, who swam 100m in under a minute, wins the award for most improved swimmer."