In stan function reference LKJ Cholesky is described in the following form:

\mathrm{LkjCholesky}(L|\eta)\propto |J|\mathrm{det}(LL^\mathsf{T})^{\eta-1}=\prod_{k=2}^KL_{kk}^{K-k+2\eta-2}

However I cannot find the explanation what is J. Can someone help?