The term 'Geologic Span' refers to the time scale of Earth's history, which is divided into four main categories: EPOCH (5 letters), ERA (3 letters), EON (3 letters), and EONS (4 letters). An EPOCH is a period of time lasting several million years, while an ERA is a longer period of time lasting hundreds of millions of years. An EON is the longest period of time, lasting billions of years, and EONS is the plural form of EON. Together, these four categories make up the Geologic Span, which is used to measure the age of Earth and its geological features.