Definition:Independent Random Variables/Dependent

Let $\EE$ be an experiment with probability space $\struct {\Omega, \Sigma, \Pr}$.
Let $X$ and $Y$ be random variables on $\struct {\Omega, \Sigma, \Pr}$.
Then $X$ and $Y$ are defined as dependent (on each other) if and only if $X$ and $Y$ are not independent (of each other).