The domain of a quadratic function, such as \( y = x^2 - 2x - 15 \), consists of all possible values of \( x \) for which the function is defined. Since a quadratic function is defined for all real numbers, the domain of this function is:
\[ \text{Domain: } (-\infty, \infty) \]
This means you can input any real number for \( x \) in the function.