What is a word to which preffixes may be added create related word?

A word to which prefixes may be added to create related words is called a base word or root word.

Here's why:

* Base Word: This is the fundamental part of a word that carries the core meaning. Prefixes are added to the beginning of the base word to modify its meaning.

* Root Word: This term is often used interchangeably with base word, although it can also refer to a word that has been borrowed from another language.

Example:

* Base Word: "happy"

* Prefixes: "un-", "over-", "pre-"

* Related Words: "unhappy," "overjoyed," "preoccupied"

Let me know if you'd like more examples!