A base word is the part of a word where the basic meaning is contained. This is commonly referred to as a "root" word.
A word can have three parts: a prefix, a suffix and a root word. The prefix is the part added to the front of the root word, while the suffix is added to the end. In most cases of simple words, only the root can stand alone as a word. An example of a root word would be "suddenly," where "sudden" is the root, meaning "occurring without warning," and the suffix is "ly," which is not a word by itself, and is used to turn a root word into an adjective.