To evaluate the expression \( \sqrt{19} - \frac{3}{2} \):
- The term \( \sqrt{19} \) is an irrational number because it cannot be expressed as a fraction of two integers.
- The term \( \frac{3}{2} \) is a rational number because it can be expressed as a fraction of two integers.
When you subtract a rational number from an irrational number, the result is always irrational.
Therefore, the answer is:
an irrational number.