A prefix is an affix which is placed before the stem of a word. Adding it to the beginning of one word changes it into another word. For example, when the prefix un- is added to the word happy, it creates the word unhappy. Particularly in the study of languages, a prefix is also called a preformative, because it alters the form of the words to which it is affixed.
Prefixes, like other affixes, can be either inflectional, creating a new form of the word with the same basic meaning and same lexical category (but playing a different role in the sentence), or derivational, creating a new word with a new semantic meaning and sometimes also a different lexical category. Prefixes, like all other affixes, are usually bound morphemes.
This is a fairly comprehensive, although not exhaustive, list of derivational prefixes in English. Depending on precisely how one defines a derivational prefix, some of the neoclassical combining forms may or may not qualify for inclusion in such a list. This list takes the broad view that acro- and auto- count as English derivational prefixes because they function the same way that prefixes such as over- and self- do.
As for numeral prefixes, only the most common members of that class are included here. There is a large separate table covering them all at .
The choice between hyphenation or solid styling for prefixes in English is covered at Hyphen > Prefixes and suffixes.
As a part of the formation of nouns, prefixes are less common in Russian than suffixes, but alter the meaning of a word.
In German, derivatives formed with prefixes may be classified in two categories: those used with substantives and adjectives, and those used with verbs. 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, and cannot simply be added to any noun or adjective.
Verbal prefixes commonly in use are be-, er-, ent-, ge-, ver-, zer-, and miss- (see also Separable verb). be- expresses strengthening or generalization. ent- expresses negation. ge- indicates the completion of an action, and that's 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. 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-.
The prefix er- usually indicates the successful completion of an action, and sometimes the conclusion means death. With fewer verbs, it indicates the beginning of and action. The prefix er- is also used to form verbs from adjectives (e.g. erkalten is equivalent to kalt werden which means to get cold).