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ukulele a musical instrument that looks like a small guitar with four strings. The ukulele was brought from Portugal to Hawaii, where it became very popular.
ulcer an open sore on the body. Ulcers produce pus and are found on the skin or in the stomach or small intestine.
ultra- a prefix that means "above" or "beyond." [2 definitions]
umbilical cord a cord that connects an unborn baby to its mother. The umbilical cord supplies nourishment to the fetus and takes away wastes.
umbrella a screen made of fabric stretched over a folding frame. An umbrella is used to shield against the rain or sun.
umpire a person who rules on plays in a game; referee. [2 definitions]
UN an abbreviation for United Nations.
un- a prefix that means "not." [2 definitions]
unable not having the power, skill, or means needed to do something; not able.
unanimous in complete agreement. [2 definitions]
unaware not aware; not realizing. [2 definitions]
unawares by surprise or in a way that was not expected.
unbeaten not defeated or outdone. [2 definitions]
unbecoming not attractive or proper on a particular person. [2 definitions]
unbelievable astounding or very hard to believe.
uncanny beyond ordinary human or natural means or ability.
uncertain not known for sure; not definite. [3 definitions]
uncle the brother or brother-in-law of one's father or mother. [2 definitions]
unclear hard to understand; confusing. [2 definitions]
Uncle Sam a cartoon person that represents the government or people of the United States. Uncle Sam is a tall, thin man with a beard, top hat, and trousers that have red and white stripes. [2 definitions]
uncomfortable feeling discomfort; not comfortable. [2 definitions]