A memoir is any nonfiction narrative writing based on the author's personal memories. The assertions made in the work are thus understood to be factual. While memoir has historically been defined as a subcategory of biography or autobiography since the late 20th century, the genre is differentiated in form, presenting a narrowed focus, usually a particular time phase in someone's life or career. A biography or autobiography tells the story "of a life", while a memoir often tells the story of a particular career, event, or time, such as touchstone moments and turning points in the author's life. The author of a memoir may be referred to as a memoirist or a memorialist.