An "affix" is a linguistic element that is added to a word to modify its meaning or create a new word. Affixes can be prefixes, which are added to the beginning of a word, or suffixes, which are added to the end. For example, in the word unhappy, "un-" is a prefix that negates the meaning of the root word "happy." Affixes play a crucial role in the formation of words in many languages, including English. They help in creating variations of a word, such as changing a noun to an adjective or a verb to a noun. Understanding affixes can enhance vocabulary and improve language skills.