What defines a prefix in word formation?

A prefix is defined as a letter or group of letters that is added to the beginning of a base word to modify its meaning. This addition helps to create new words by altering the original word in a meaningful way. For example, when you take the base word "happy" and add the prefix "un-", it transforms into "unhappy," indicating the opposite of what the original word expressed. This illustrates how prefixes serve as a way to expand vocabulary by changing the meaning of base words.

In contrast, adding a letter to the end of a word refers to a suffix, which functions differently in word formation. Changes in spelling of a base word can be a result of various morphological processes, such as inflections or derivations, but this does not specifically define a prefix. Additionally, while base words are essential components of word formation, a letter that serves as a base word does not capture the essence of what prefixes do in the context of language structure.