A prefix is an affix that is added to the beginning of a base word to slightly change its meaning. Prefixes are one to three syllables long and are a regular part of English. They can be either inflectional or derivational. Inflectional prefixes create a new form of a word with the same basic meaning and same lexical category, while derivational prefixes create a new word with a new semantic meaning and sometimes also a different lexical category.

Prefixes are used mainly to shorten phrases into a single word. They can be used to create new words in English, make a word negative, or express relations of time, place, or manner. For example, adding the prefix "im-" to the base word "possible" creates a new word, "impossible," which means "not possible".

Some common prefixes in English include "un-," "dis-," "pre-," "re-," and "in-". Unlike suffixes, prefixes dont change the spelling of the root word when added. Students should learn the most common prefixes early, as knowing what prefixes mean can help them make sense of unknown words.

In summary, a prefix is a word part added to the beginning of a word or base word to modify its meaning.