un-

from the GNU version of the Collaborative International Dictionary of English.

• An inseparable prefix, or particle, signifying not; in-; non-. In- is prefixed mostly to words of Latin origin, or else to words formed by Latin suffixes; un- is of much wider application, and is attached at will to almost any adjective, or participle used adjectively, or adverb, from which it may be desired to form a corresponding negative adjective or adverb, and is also, but less freely, prefixed to nouns. Un- sometimes has merely an intensive force; as in unmerciless, unremorseless.
• Un- is prefixed to adjectives, or to words used adjectively.
• To adjectives, to denote the absence of the quality designated by the adjective.
• To past particles, or to adjectives formed after the analogy of past particles, to indicate the absence of the condition or state expressed by them.
• An inseparable verbal prefix or particle. It is prefixed: (a) To verbs to express the contrary, and not the simple negative, of the action of the verb to which it is prefixed; as in unbend, uncoil, undo, unfold. (b) To nouns to form verbs expressing privation of the thing, quality, or state expressed by the noun, or separation from it; as in unchild, unsex. Sometimes particles and participial adjectives formed with this prefix coincide in form with compounds of the negative prefix un- (see 2d un-); as in undone (from undo), meaning unfastened, ruined; and undone (from 2d un- and done) meaning not done, not finished. Un- is sometimes used with an intensive force merely; as in unloose.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• prefix not
• prefix absent, lacking, not
• prefix Used to form temporary names of elements (such as ununbium) whose existence has been predicted, and have not yet been given a systematic name.
• prefix reverse, opposite
• prefix release, free, remove, extract.