Commonly and also acceptably defined in terms of the overlap (or 'valley') between two peaks. Thus for two peaks of equal height, masses $m_1$ and $m_2$, when there is overlap between the two peaks to a stated percentage of either peak height (10% is recommended), then the resolving power is defined as $\frac{m_1}{m_1-m_2}$. The percentage overlap (or 'valley') concerned must always be stated.