Periods of history are divided into ERAS, AGES, and EPOCHS. An era is a long period of time marked by certain characteristics or events, such as the Middle AGES or the Industrial Revolution. An age is a shorter period of time, usually within an era, that is distinguished by certain features, such as the Stone Age or the Iron Age. An epoch is a very specific period of time, often within an age, that is defined by a particular event or development, such as the Neolithic Revolution or the Enlightenment.