The term 'Geologic Units of Time' refers to the four major divisions of geologic time: ERAS, Periods, Epochs, and Ages. ERAS are the largest units of geologic time, spanning millions of years. They are divided into smaller units of time, such as Periods, Epochs, and Ages. ERAS are typically divided into two or more Periods, which are further divided into Epochs and Ages. Each of these units of time is used to describe the Earth's history and the evolution of life on Earth.