Use this tag if you are asking about the Bhattacharyya distance or the Bhattacharyya coefficient.
The Bhattacharyya coefficient $(1)$ and is the Bhattacharyya distance $(2)$ are:
\begin{align} BC(\mathbf{p},\mathbf{q}) &= \sum_{i=1}^n \sqrt{p_i q_i} \tag{1} \\[5pt] D_B(p,q) &= -\ln \left( BC(p,q) \right) \tag{2} \end{align}
The Bhattacharyya distance is a distance measure similar to Total Variation distance or Hellinger distance.