To determine the undefined value for the equation \( \frac{8}{x} - 15 = 13 \), we need to find what \( x \) cannot equal.
The term \( \frac{8}{x} \) becomes undefined when the denominator \( x \) is equal to 0. Therefore, the equation is undefined when:
\[ x = 0 \]
So, the answer is:
0