In information theory, the conditional entropy quantifies the amount of information needed to describe the outcome of a random variable Y given that the value of another random variable X, X is known. Here, information is measured in shannons, nats, or hartleys. The entropy of Y conditioned on X is written as .
Remember, the entropy means the uncertainty of the variable. Thus the conditional entropy means the uncertainty of when knowing .
\begin{align*} \mathcal{H}(Y|X) &= - \sum_{x,y} p(x, y) \log p(y|x) \\ &= - \sum_{x,y} p(x,y) \log \frac{p(x,y)}{p(x)} \end{align*}