The term 'PERIODS IN HISTORY BOOKS' can be summarized as 'ERAS'. An era is a period of time in history that is marked by certain characteristics or events. It is often used to divide up long stretches of time into more manageable chunks for study and analysis. ERAS are typically divided into smaller units such as centuries, decades, or years. Examples of ERAS include the Stone Age, the Renaissance, and the Industrial Revolution.