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Definition 2024

joint entropy ‎(plural joint entropies)

(information theory) The Shannon entropy of a "script" whose "characters" are elements of the Cartesian product of the sets of characters of the component scripts.
If random variables $X$ and $Y$ are mutually independent, then their joint entropy $H(X,Y)$ is just the sum $H(X)+H(Y)$ of its component entropies. If they are not mutually independent, then their joint entropy will be $H(X)+H(Y)-I(X;Y)$ where $I(X;Y)$ is the mutual information of $X$ and $Y$ .