What is Kröger–Vink notation?

Kröger–Vink notation writes a point defect as $\mathrm{A_S^c}$: A is the species on the site (V for a vacancy, $i$ as the site for an interstitial), S the site it occupies in the perfect crystal, and c its charge relative to that site: $\bullet$ for each positive unit, $'$ for each negative unit, $\times$ for none.