A prefix is a group of letters added to the beginning of a word to change its meaning. For example, the prefix un- can be added to the word happy to create the word unhappy, which means not happy. Prefixes can indicate negation, repetition, or a change in state, among other things. Prefixes are commonly used in the English language and can help expand vocabulary. Other examples include re-, which means again, as in redo, and pre-, which means before, as in preview. Understanding prefixes can make it easier to decipher unfamiliar words.