A prefix is an affix which is placed before the stem of a word.^[1] Particularly in the study of languages, a prefix is also called a preformative, because it alters the form of the word to which it is affixed.

Prefixes, like other affixes, can be either inflectional, creating a new form of a word with the same basic meaning and same lexical category, or derivational, creating a new word with a new semantic meaning and sometimes also a different lexical category.^[2] Prefixes, like all affixes, are usually bound morphemes.^[1]

English has no inflectional prefixes, using only suffixes for that purpose. Adding a prefix to the beginning of an English word changes it to a different word. For example, when the prefix un- is added to the word happy, it creates the word unhappy.

The word prefix is itself made up of the stem fix (meaning "attach", in this case), and the prefix pre- (meaning "before"), both of which are derived from Latin roots.

Commonly used prefixes in Japanese include お〜 (o-) and ご〜 (go-). They are used as part of the honorific system of speech, and are used as markers for politeness, showing respect for the person or thing they are affixed to, notably also being used euphemistically.^[3][4]

In the Bantu languages of Africa, which are agglutinating, the noun class is conveyed through prefixes, which is declined and agrees with all of its arguments accordingly.^[5]

Verbs in the Navajo language are formed from a word stem and multiple affixes. For example, each verb requires one of four non-syllabic prefixes (∅, ł, d, l) to create a verb theme.^[6]

In the Sunwar language of Eastern Nepal, the prefix ma- म is used to create negative verbs. It is the only verbal prefix in the language.

As a part of the formation of nouns, prefixes are less common in Russian than suffixes, but alter the meaning of a word.

пред- and положение 'position' becomes предположение 'supposition'

пре- and образование 'formation (verb)' becomes преобразование 'transformation'[8]

In German, derivatives formed with prefixes may be classified in two categories: those used with substantives and adjectives, and those used with verbs.^[9] For derivative substantives and adjectives, only two productive prefixes are generally addable to any substantive or adjective as of 1970: un-, which expresses negation (as in ungesund, from gesund), and ur-, which means "original, primitive" in substantives, and has an emphatic function in adjectives. ge-, on the other hand, expresses union or togetherness, but only in a closed group of words—it cannot simply be added to any noun or adjective.^[10]

Verbal prefixes commonly in use are be-, ent-, er-, ge-, miss-, ver-, and zer- (see also Separable verb).^[10] be- expresses strengthening or generalization. ent- expresses negation. ge- indicates the completion of an action, which is why its most common use has become the forming of the past participle of verbs; ver- has an emphatic function, or it is used to turn a substantive or an adjective into a verb.^[9] In some cases, the prefix particle ent- (negation) can be considered the opposite of particle be-, while er- can be considered the opposite of ver-.^[11][12]

The prefix er- usually indicates the successful completion of an action, and sometimes the conclusion means death.^[13] With fewer verbs, it indicates the beginning of an action.^[9][13] The prefix er- is also used to form verbs from adjectives (e.g. erkalten is equivalent to kalt werden which means "to get cold").^[13]

• Affix
• Suffix
• Privative
• Bound and unbound morphemes
• English prefix
• List of Greek and Latin roots in English
• substring#Prefix
• Metric prefix