Meaning: one, single
* Unicycle: A cycle with one wheel.
* Unicorn: A mythical creature with one horn.
* Unilateral: Affecting only one side.
* Unify: To make into one.
* Unique: Being the only one of its kind.
* Unison: In complete agreement or harmony.
* Universe: All existing matter and space considered as a whole.
* Unit: A single, distinct entity.
* Unity: The state of being united or joined as one.
Meaning: not, opposite of
* Unbelievable: Impossible to believe.
* Unclear: Not clear or definite.
* Uncommon: Not common or usual.
* Unconscious: Not aware of one's surroundings.
* Unfair: Not just or equal.
* Unhappy: Not happy or content.
* Unhealthy: Not healthy or well.
* Unimportant: Not important or significant.
* Unlikely: Not likely to happen.
* Unload: To remove a load from something.
* Unlock: To open something that is locked.
* Unplug: To disconnect something from an electrical source.
* Unwrap: To remove the wrapping from something.
Other meanings:
* University: An institution of higher education.
* Unisex: Suitable for both sexes.
* Unidentified: Not known or recognized.
* Unintended: Not planned or intended.
* Uninterrupted: Without interruption or break.
* Unlimited: Without limits or bounds.
* Unpredictable: Not able to be predicted.
* Unqualified: Not having the necessary qualifications.
* Unrealistic: Not realistic or practical.
This is not an exhaustive list, but it gives you a good starting point. Remember, prefixes can be combined with other prefixes or suffixes to create more complex words.