The student council president, a popular senior, led the parade.

Appositive:

Noun or pronoun defined:

Appositive: a popular senior

Noun or Pronoun defined: President

Explanation:

Appositive is a phrase that renames another phrase. Both phrases are placed side by side separated by a comma. For example, if we say, Mrs. Kumar, the English teacher, is on leave today. "the English teacher" is defining Mrs. Kumar, so it is an appositive.

In the given sentence, "a popular senior" is defining or renaming "the student council president', so this is an appositive. The noun or pronoun which is defined is "The student council President".