The set of numbers that are real numbers but cannot be expressed as one integer divided by another is called irrational numbers. Irrational numbers cannot be expressed as a fraction of two integers (i.e., they cannot be written in the form \( \frac{p}{q} \) where \( p \) and \( q \) are integers and \( q \neq 0 \)). Examples of irrational numbers include numbers like \(\sqrt{2}\), \(\pi\), and \(e\).
So the correct response is: irrational.