u

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The 21st letter of the modern English alphabet.
• noun Any of the speech sounds represented by the letter u.
• noun The 21st in a series.
• noun Something shaped like the letter U.
• noun A grade that indicates an unsatisfactory status.
• abbreviation up quark

from The Century Dictionary.

• noun An abbreviation of University College; of Upper Canada; of the Latin urbis conditæ, from the founding of the city, moaning from the first year of Rome.
• noun An abbreviation of United Brethren; of United Brethren in Christ.
• The twenty-first character and fifth vowel-sign in the English alphabet.
• As a symbol: The chemical symbol of uranium.
• In quaternions, an operational sign which, prefixed to the symbol of a quaternion, denotes the versor of that quaternion.
• In the theory of heat, a symbol used to denote the energy, or the sum of the increment of heat and the heat consumed.
• [lowercase] In the calculus, the symbol of a function.
• [lowercase] In hydrodynamics, used with v and w to denote the rectangular components of the velocity.
• noun An abbreviation of United States (of America).
• noun An abbreviation of Uncle Sam
• noun of United Service
• noun [lowercase or cap.] of the Latin ut supra, as above.
• An abbreviation of underproof, as applied to alcoholic liquors.
• An abbreviation [l. c] in a ship's log-book, of ugly threatening weather; [l. c] of uncle; of Unionist.
• noun An abbreviation of Italian una corda, on one string.
• noun An abbreviation of United Presbyterian.
• noun An abbreviation of United Kingdom (of Great Britain and Ireland).

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

• the twenty-first letter of the English alphabet, is a cursive form of the letter V, with which it was formerly used interchangeably, both letters being then used both as vowels and consonants. U and V are now, however, differentiated, U being used only as a vowel or semivowel, and V only as a consonant. The true primary vowel sound of U, in Anglo-Saxon, was the sound which it still retains in most of the languages of Europe, that of long oo, as in tool, and short oo, as in wood, answering to the French ou in tour. Etymologically U is most closely related to o, y (vowel), w, and v; as in two, duet, dyad, twice; top, tuft; sop, sup; auspice, aviary. See v, also o and y.

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

• noun metrology symbol for unified atomic mass unit
• noun phonetics Used in the International Phonetic Alphabet and in several romanization systems of non-Latin scripts to represent a close back rounded vowel (IPA: /u/).
• noun The twenty-first letter of the basic modern Latin alphabet.
• noun The twenty-first letter of the English alphabet, called u and written in the Latin script.
• noun The name of the Latin script letter U/u.
• noun A thing in the shape of the letter U
• pronoun abbreviation, slang, text messaging, Internet you (in text messaging and internet conversations)
• abbreviation Underwater.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective (chiefly British) of or appropriate to the upper classes especially in language use
• noun a base containing nitrogen that is found in RNA (but not in DNA) and derived from pyrimidine; pairs with adenine
• noun a heavy toxic silvery-white radioactive metallic element; occurs in many isotopes; used for nuclear fuels and nuclear weapons
• noun the 21st letter of the Roman alphabet