U.K. Acceptable as a noun or adjective in all references for United Kingdom. Use without periods in headlines.

U.N. Acceptable in references for United Nations if the meaning is clear in the context. Use without periods in headlines.

ultra- In general, no hyphen when used as a prefix: ultraconservative, ultramodern, ultrasonic, ultraviolet. See Words Formed with Prefixes under General Style Preferences.

un- In general, no hyphen when used as a prefix, unless it is followed by a proper name: unarmed, unnecessary, unprepared, unshaven; un-American. See Words Formed with Prefixes under General Style Preferences.

unique Reserve this word for the sense “one of a kind.” Avoid it in the sense “special, unusual.” Phrases such as very unique, more unique, somewhat unique, and so on—in which a degree is attributed to unique—are poor usage.

U.S. Acceptable in all uses for United States. Use without periods in headlines.

USA No periods in the abbreviated form for United States of America.

use, usage, utilize Use is usually the best choice for simplicity. Usage refers to a customary practice. Utilize is occasionally a better choice than use when the distinct sense is “to use to best effect.”