Negation; reversal of a state.
[Old English prefixes un- and on-, of Germanic origin.]
This prefix occurs extremely widely; the majority of adjectives, adverbs, and verbs (and many nouns) can in principle be given it to create a new word indicating an opposite or a reversal.
A very few of the many examples in the sense of ‘not’ are unconnected, unenclosed, unfashionable, unhappy, unloved, unmade, unsuitable, and unwilling. In this sense, un- often has a stronger and less neutral force than just negation (so it is not equivalent to non-): unkind can mean active cruelty rather than a simple lack of kindness; to say someone is un-American can imply an active antagonism to American ways.
With verbs, it usually has the sense of reversing some state: unblock, unburden, unhook, unlace, unsettle, unstick, untie, unwind, unzip.
Un- is closely related in sense to in- (see in-1), but although the latter prefix is common it is no longer active. There is no good rule to decide which is the right form in any given situation and terms have to be learnt. To confuse matters somewhat, some noun-adjective pairs use different prefixes: instability corresponds to unstable; inequality to unequal; injustice to unjust. In a few cases, pairs of adjectives exist in both prefixes with similar senses: inadvisable and unadvisable; incommunicative and uncommunicative. In a few other pairs, members have significantly different senses, as with unhuman, not resembling or having the qualities of a human being, versus inhuman, lacking human qualities of compassion and mercy.