The notation "ℝ \ ℚ" represents the set of all real numbers that are not rational numbers. In this context, ℝ stands for the set of real numbers, which includes both rational and irrational numbers, while ℚ denotes the set of rational numbers. Rational numbers can be expressed as the quotient of two integers, whereas irrational numbers cannot be written in such a form. The set "ℝ \ ℚ" specifically includes numbers like π and √2, which cannot be expressed as fractions. These numbers have non-repeating, non-terminating decimal expansions, distinguishing them from rational numbers. Thus, "ℝ \ ℚ" is the collection of all irrational numbers.