A privative, named from Latin privare, "to deprive", is a particle that negates or inverts the value of the stem of the word. In Indo-European languages many privatives are prefixes; but they can also be suffixes, or more independent elements.
- un- from West Germanic; e.g. unprecedented, unbelievable
- in- from Latin; e.g. incapable, inarticulate.
- a-, called alpha privative, from Ancient Greek α-, αν-; e.g. apathetic, abiogenesis.
These all stem from a PIE syllabic nasal privative *n̥-, the zero ablaut grade of the negation *ne, i.e. "n" used as a vowel, as in some English pronunciations of "button". This is the source of the 'n' in 'an-' privative prefixed nouns deriving from the Greek, which had both. For this reason, it appears as an- before vowel, e.g. anorexia, anesthesia.
The same prefix appears in Sanskrit, also as a-, an-. In in Slavic languages the privative is nie- and u-, e.g. nieboga, ubogi. In North Germanic languages, the -n- has disappeared and Old Norse has ú- (e.g. ú-dáins-akr), Danish and Norwegian have u-, whereas Swedish uses o, and Icelandic uses the etymologically related ó.
Confusion of privative and non-privative in English
Many words introduced into the English from the Latin start with the prefix in-. While often, it is a privative, it is not always so. Even if it is a privative, the meaning may be unclear to those who are not familiar with the word. The following three examples illustrate that.
Example 1: inexcusable
The prefix is a privative and the word means the opposite of excusable that is, "unable to be excused, not excusable".
Example 2: invaluable
That is also a privative but it does not mean "not valuable, not precious". While today valuable is a synonym for precious, it originally meant "able to be given a value". The meaning of invaluable hinges upon this original meaning and thus means "of very great value" or literally "value cannot be estimated (because it is so great)", similar to priceless but dissimilar to worthless.
Example 3: inflammable
A naive reader may interpret this as "not flammable". However, that is wrong. The word contains not a privative but a locative and flammable (the newer word in English by three hundred years) rather than being the opposite of inflammable means the same thing.
Here the prefix in- arises from the Latin for "in, inside, within" and inflammable derives from the Latin root inflammāre meaning "able to be set alight, able to kindle a flame". Since at least the 1920s, there have been calls to stop using inflammable and substitute it exclusively with flammable to avoid confusion that happens even with native English-speakers. 
- privare, at William Whitaker's Words.
- http://michiganradio.org/post/you-re-some-confusion-prefixes retrieved 14 may 2014
- http://dictionary.reference.com/browse/invaluable?s=t, retrieved 13 May 2014
- http://grammar.about.com/od/alightersideofwriting/a/Flammable-Inflammable-And-Nonflammable.htm retrieved 14 may 2014
- http://dictionary.reference.com/browse/inflamed?s=t, retrieved 13 May 2014
^ The meaning "able to be given a value" is largely obsolete today.