A prefix is an affix which is placed before the stem of a word.
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.
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.
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
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.
it alters the form of the words to which it is affixed.

Prefixes, like other affixes, can be either inflectional, changing the syntactic category,
Prefixes, like other affixes, can be either inflectional, changing the syntactic category,

or derivational, changing either the lexical category or the semantic meaning.
or derivational, changing either the lexical category or the semantic meaning.

In English, there are no inflectional prefixes.
In English, there are no inflectional prefixes.

Prefixes, like all other affixes, are bound morphemes.
Prefixes, like all other affixes, are bound morphemes.

The word prefix is itself made up of the stem fix, and the prefix pre-, both of which are
The word prefix is itself made up of the stem fix, and the prefix pre-, both of which are

derived from Latin roots.
derived from Latin roots.

In English List of English derivational prefixes
In English List of English derivational prefixes

In English, a fairly comprehensive list, although not exhaustive, is the following.
In English, a fairly comprehensive list, although not exhaustive, is the following.

Depending on precisely how one defines a derivational prefix, some of the neoclassical combining
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.
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
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.
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.
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 Numeral prefix > Table of number prefixes
There is a large separate table covering them all at Numeral prefix > Table of number prefixes

in English.
in English.

Hyphenation The choice between hyphenation or solid styling
Hyphenation The choice between hyphenation or solid styling

for prefixes in English is covered at Hyphen > Prefixes and suffixes.
for prefixes in English is covered at Hyphen > Prefixes and suffixes.

In other languages Japanese
In other languages Japanese

The most commonly used prefix in Japanese, お o-, is used as part of the honorific system
The most commonly used prefix in Japanese, お o-, is used as part of the honorific system

of speech.
of speech.

It is a marker for politeness, showing respect for the person or thing it is affixed to.
It is a marker for politeness, showing respect for the person or thing it is affixed to.

Bantu languages In the Bantu languages of Africa, which are
Bantu languages In the Bantu languages of Africa, which are

agglutinating, the noun class is conveyed through prefixes, which is declined and agrees
agglutinating, the noun class is conveyed through prefixes, which is declined and agrees

with all of its arguments accordingly.
with all of its arguments accordingly.

Example from Ganda The one, old, fat farmer goes.
Example from Ganda The one, old, fat farmer goes.

Navajo Verbs in the Navajo language are formed with
Navajo Verbs in the Navajo language are formed with

a stem and multiple affixes.
a stem and multiple affixes.

For example, each verb requires one of four non-syllabic prefixes to create a verb theme.
For example, each verb requires one of four non-syllabic prefixes to create a verb theme.

Sunwar In the language of the Sunuwar people of Eastern
Sunwar In the language of the Sunuwar people of Eastern

Nepal, the prefix ma- म is used to create negative verbs.
Nepal, the prefix ma- म is used to create negative verbs.

It is the only verbal prefix in the language.
It is the only verbal prefix in the language.

Bad child!
Bad child!

Russian As a part of the formation of nouns, prefixes
Russian As a part of the formation of nouns, prefixes

are less common in Russian than suffixes, but alter the meaning of a word.
are less common in Russian than suffixes, but alter the meaning of a word.

German In German, derivatives formed with prefixes
German In German, derivatives formed with prefixes

may be classified in two categories: those used with substantives and adjectives, and
may be classified in two categories: those used with substantives and adjectives, and

those used with verbs.
those used with verbs.

For derivative substantives and adjectives, only two prefixes are still in use as of 1970:
For derivative substantives and adjectives, only two prefixes are still in use as of 1970:

un-, which expresses negation, and ur-, which means "original, primitive" in substantives,
un-, which expresses negation, and ur-, which means "original, primitive" in substantives,

and has an emphatic function in adjectives.
and has an emphatic function in adjectives.

ge- expresses union or togetherness.
ge- expresses union or togetherness.

On the other hand, verbal prefixes are still much in use: be-, er-, ent-, ge-, ver-, zer-,
On the other hand, verbal prefixes are still much in use: be-, er-, ent-, ge-, ver-, zer-,

and miß-. be- expresses strengthening or generalization.
and miß-. be- expresses strengthening or generalization.

ent- expresses negation.
ent- expresses negation.

ge- indicates the completion of an action, and that's why its most common use has become
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
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.
to turn a substantive or an adjective into a verb.

In some cases, the prefix particle ent- can be considered the opposite of particle be-,
In some cases, the prefix particle ent- can be considered the opposite of particle be-,

while er- can be considered the opposite of ver-.
while er- can be considered the opposite of ver-.

The prefix er- usually indicates the successful completion of an action, and sometimes the
The prefix er- usually indicates the successful completion of an action, and sometimes the

conclusion means death.
conclusion means death.

With fewer verbs, it indicates the beginning of and action.
With fewer verbs, it indicates the beginning of and action.

The prefix er- is also used to form verbs from adjectives.
The prefix er- is also used to form verbs from adjectives.

See also Affix
See also Affix

Suffix Bound morpheme
Suffix Bound morpheme

English prefixes List of Greek and Latin roots in English
English prefixes List of Greek and Latin roots in English

Prefix — the mathematical notion in formal language theory
Prefix — the mathematical notion in formal language theory

References
References