Yes, 18 is a rational number. A rational number is defined as a number that can be expressed as the quotient or fraction of two integers, where the denominator is not zero.
In the case of 18, it can be written as 18/1, which clearly shows it as a fraction where both the numerator (18) and the denominator (1) are integers. Since it meets the criteria of being expressed as a fraction of integers, we can confidently say that 18 belongs to the set of rational numbers.