Universal Set

This is the set from which all the elements being examined are members. The universal set is denoted by the symbol U.

Example
Using set builder notation, where {x:..} means ‘the set of all x such that’,
If A = {0, 1, 2, 3, 4, 5, 6…}
Then U = {x: x≥ 0, x Є W}

That is, the universal set, U = W, the set of whole numbers.