The term 'HISTORY CLASS SUBJECT' can be summarized as the study of past events and eras. The crossword solution for this term is 'ERA, ' which is a three-letter word that refers to a period of time in history. History classes typically focus on the study of different eras, such as the Ancient Greek ERA, the Middle Ages, the Renaissance, and the Industrial Revolution. By studying these eras, students gain an understanding of how societies have changed over time and how they have been shaped by the events of the past.