Appositive is a noun or noun phrase that <em>renames a noun beside it</em>, usually a noun that comes before it in a sentence. Appositives can be <em>essential</em> (it contains information necessary for the meaning of the sentence.) or <em>nonessential</em> (the information is not needed for the complete meaning of the sentence.) <em><u>Galileo</u></em> is necessary because it helps to know <em>what "famed astronomer" the writer is talking about.</em>