Psychologists, who are working in the broad area of research on emotion, need clearer definitions of the emotion-related words that are used as technical terms. This article reports the frequency of occurrence of six conceptual words that have appeared in historically important writings. The words that were analyzed were: affection, emotion, feeling, passion, sensibility, and sentiment. The study surveyed the following seven books: Descartes' “The passions of the soul,” Spinoza's “Ethica,” Hume's “A treatise of human nature (Book 2), ” Adam Smith's “The theory of moral sentiments,” Haven's “Mental philosophy (Division 2), ” and James' “The principles of psychology (Chapter 25), ” and “Psychology, briefer course (Chapter 24).” In the 17th and 18th centuries the word “passion (s)” was preferred in the articles. In the 19th century, however, American psychologists mostly used emotion (s) rather than passion (s).