the reason or intent the author has for writing the piece
Purpose is usually found in the identification of the type of work created. In a story, the author’s purpose is to entertain. In a persuasive essay, the author’s purpose is to sway the reader to a particular side, etc. There may be more than one purpose identified for a text. For example, the author may want to entertain the audience, but also satirize a social issue they want to see addressed.