Writing instrument refers to any tool or device that is used for writing. There are several synonyms for the term "writing instrument", including "marker", "pen", "pencil", "stylus" and "quill". Each of these words has specific connotations and is used in different situations. For example, "pen" refers specifically to a tool that uses ink, while "pencil" refers to a tool that uses graphite or other materials. "Stylus" is a term used for instruments that are used for digitizing, while "quill" is a type of pen that was widely used in the past. Regardless of the word used, writing instruments are essential for human communication and have been important tools throughout history.