What is the exact meaning of $I(X;Y|Z)$ in Information Theory?

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I am wondering: is the notation $I(X;Y|Z)$ used to denote the mutual information between probabilities of $X$ and $Y|Z$ or between $X|Z$ and $Y|Z$?

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$$ I(X;Y|Z) = \sum_{x,y,z}p(x,y,z)\log{p(x,y|z)\over p(x|z)p(y|z)} $$ so it isn't either of those things, but the mutual information between $X|Z$ and $Y|Z$ is a better description.

[Reference: equation (2.118) of Information Theory and Network Coding by R. Yeung.]