What is the set denoted by U?

First and Second definitions are both correct

Explanation

In any discussion about sets, all the sets under consideration are to be the subsets of a particular set. Such a set is called the universal set or universe of discourse. It is denoted by U.

Examples:

1. In the discussion concerning the set of odd numbers, the set of even numbers, the set of prime numbers, the set of composite numbers, the set of factors, and so on, the universal set is the set of natural numbers.
2. The set of all real numbers is the universal set in the discussion of subsets of rational and irrational numbers