The definition of author's purpose is the reason an author writes about a specific topic. After the writer chooses a topic, the author also decides if the purpose of the writing is to entertain, inform, persuade or explain the ideas to the reader.
Different authors use different styles of writing to reflect the purpose of their writing, such as the use of prose versus poetry. When determining an author's purpose, it is important to look into his background and his motivations for writing. The author may be under contract to write in a specific style or about a specific topic and have no personal investment in his writing. Conversely, the author could feel passionately about a topic and write solely out of love for a topic or idea.