We define a probability
$$ p(x_i,h_j;\boldsymbol{\Theta}) = \frac{f(x_i,h_j;\boldsymbol{\Theta})}{Z(\boldsymbol{\Theta})}, $$where \( f(x_i,h_j;\boldsymbol{\Theta}) \) is a function which we assume is larger or equal than zero and obeys all properties required for a probability distribution and \( Z(\boldsymbol{\Theta}) \) is a normalization constant. Inspired by statistical mechanics, we call it often for the partition function. It is defined as (assuming that we have discrete probability distributions)
$$ Z(\boldsymbol{\Theta})=\sum_{x_i\in \boldsymbol{X}}\sum_{h_j\in \boldsymbol{H}} f(x_i,h_j;\boldsymbol{\Theta}). $$