The description of Modern Milestones mentions “base words.” What is a base word and how is it different from a root word?

A base word is a part of a word that has meaning by itself. The meaning can be changed or expanded by adding letters to the beginning (prefix) or end (suffix) of the word. Sometimes small changes are made to the base word when endings are added. For example, the y is changed to i before the -es is added in order to change study to studies.

A root word is a part of a word that carries meaning but may not be able to stand alone. An example is the Greek root therm in thermal and thermometer.