prefix

1375–1425; (verb) late Middle English prefixen < Middle French prefixer < Latin praefixus, past participle of praefīgere “to set up in front”; see pre-, fix; (noun) < New Latin praefixum, neuter of praefixus

A prefix is one kind of affix. An affix is an element that, although not a word itself, can be bound to a word, or to the base or stem of a word, to form a derivative with a related meaning. Entire families of related words can be derived from an existing word in this way.
Although linguists sometimes divide the subject more finely, there are essentially three kinds of affix —the prefix, attached to the beginning (as un- in unconscious ), the infix, inserted in the middle, and the suffix, tacked on at the end (as -ness, in consciousness ). Note the lack of an example for infix. Standard English—unlike some other languages—does not use infixes, unless we count isolated instances of tmesis, where instead of an affix, an entire word is inserted, as in the playful “abso-bloomin'-lutely” in the lyrics of “Wouldn't It Be Loverly?”—a song from the musical My Fair Lady. (“Bloomin'” is probably a euphemism; other tmetic insertions are more often than not obscene.) But one-shot nonce words such as “abso-bloomin'-lutely” are neither added to the language nor found in standard dictionaries of English. On the other hand, both prefixes and suffixes are highly productive derivational forms, constantly in use to form new English words.
A few of the most productive English prefixes are anti-, non-, pre-, re-, sub-, and un-. They are so common that some print dictionaries show simple lists, without definitions, of words that have been formed with them—taking it for granted that their meanings are obvious. Here at Dictionary.com, however, such terms, when not given entries and definitions of their own, are shown with other related forms at the bottom of the entry for the word on which the derivative is based. Thus a query for unacetic will take you automatically to acetic, helping you to understand your queried word without having to look up its bits and pieces separately.
The grammatical term prefix itself has the prefix pre-; in this case, pre- means “before; preceding” and one meaning of fix is “to attach or place.” A word can have more than one prefix, like un- and re- in unremarkable. And a prefix can be used in combination with one or more suffixes. A simple example is reactivate, which has the prefix re- “again” added to the verb activate. Activate, in turn, is composed of the adjective active “engaged in action” and the suffix -ate, used to form verbs. Activate means “to make something active; cause it to function.” To reactivate is to do this again. But we must exercise some care in our analysis of words that are new to us; a casual glance at word formations may be deceptive. For example, to capitulate is “to surrender.” But recapitulate does not mean “to surrender again.” It means “to summarize.” In addition, some prefixes have more than one meaning. For example, if you look up un- in Dictionary.com, you'll find two entries. The first, un-¹, means “not” (as in unafraid and unsettled ), the second, un-², reverses the meaning of the verb it is attached to, as in unzip or in the social media term unfriend.
But the most common problem with words formed with prefixes is determining whether or not to hyphenate between the prefix and the base word. Luckily, there are some guidelines. In general, there is no hyphen ( nonstarter, postcranial, unemployment, antievolution ). However, hyphenating is mandatory when the prefix is followed by a capital letter ( anti-Nazi, pre-Columbian ). We also hyphenate when a prefix that ends in a or i precedes a word starting with the same letter ( ultra-abysmal, anti-inflation ). However, words formed with prefixes ending in e or o followed by the same letter are in a state of flux ( cooperate and co-operate are variants, as are preeminent and pre-eminent ), but the solid form is increasingly more frequent. At the same time, certain words must be hyphenated to avoid ambiguity or misreading. re-sign (to sign again) is different from resign (to give up an office or position), and re-ink (to apply ink again) would look like some strange one-syllable word if spelled reink.
But the easiest rule to remember may be this: when you are not sure whether to hyphenate a particular word, it is best to look it up.