UN, a prefix or inseparable preposition, un or on, usually un, an, is the same word as the L. in. It is a particle of negation, giving to words to which it is prefixed, a negative signification. We use un or in indifferently for this purpose; and the tendency of modern usage is to prefer the use of in, in some words, where un was formerly used. Un admits of no change of n into l, m or r, as in does, in illuminate, immense, irresolute. It is prefixed generally to adjectives and participles, and almost at pleasure. In a few instances, it is prefixed to verbs, as in unbend, unbind, unharness. As the compounds formed with un are so common and so well known, the composition is not noticed under the several words. For the etymologies, see the simple words.