Quite simply, “tertiary” means “third.” But it also has specialized definitions in many applications. To geologists, “Tertiary” is a specific period of the Cenozoic era. Chemists use the term to describe particular organic compounds, and ecologists describe carnivores that eat other carnivores as tertiary. In the United States, childhood education is called “elementary school,” and middle school and high school are sometimes called “secondary school.” These years of education are required, but going to college or a university is optional. In the U.K., the latter is called “tertiary education.”