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 Uncountable A set is uncountable if there is no one-to-one correspondence between the elements of the set and the natural numbers. The set of real numbers is uncountable. Union The union of two given sets A & B is the set of all elements that are belonging to either A or B or both. For example,let A = (-1, 5) and B = (0, 6) be two open intervals of real numbers, then the union of A and B is , which is the set of all real numbers between -1 and 6. Unique Being the only value satisfying some condition. Unique factorization theorem Another name for the Fundamental Theorem of Arithmetic and it  states that any counting number is either a prime number or a composite number that can be factored as a product of prime numbers in a unique way, except for the order in which they are written. Unit The first position in a place-value counting system, representing a single-digit number. Unknown Indeterminate. A letter whose value(s) is to be determined by solving an equation.