Marginalia refers to the marginal notes left on books and other important documents. Think of the passages you highlight and the arrows you pull to make important observations on any type of text. That’s marginalia. You’ve done it as you hurriedly scribbled on your college textbook as your dynamic professor lectured. You’ve done it when you circled a word in order to add its definition, or when you added a “?” to a paragraph you didn’t understand.