a vacancy
Picture a cinema where every seat is taken except one — that single empty seat is a vacancy. In a crystal a vacancy is a lattice site that should hold an atom or ion but is empty. It is the simplest and usually the most abundant point defect, and it is what makes a solid 'leaky' to moving atoms: without an empty neighbouring seat, an atom locked in a dense array has nowhere to step.
A vacancy costs energy to create — you must break the bonds that held the missing ion and carry it away (to the surface, a grain boundary, or a dislocation). Yet the crystal tolerates a population of them because they buy configurational entropy, so the equilibrium concentration climbs with temperature as exp(-E_f/kT). In an ionic ceramic a vacancy is not electrically neutral relative to the perfect lattice: pluck out an O2- and you leave behind a hole in the charge pattern that behaves like a site with a net +2 charge — in Kroger-Vink notation an oxygen vacancy is written V_O with two dots (V_O••), and a magnesium vacancy V_Mg with two primes (V_Mg''). Because charge must balance overall, vacancies in ceramics are always created in charge-neutral combinations (a Schottky set, or paired with a dopant or an electronic carrier).
Vacancies are the workhorses of ceramic kinetics. Ionic diffusion in most oxides proceeds by the vacancy mechanism — an ion and a neighbouring vacancy swap places, so the vacancy drifts one way while atoms drift the other. Oxygen vacancies specifically are the mobile charge carriers in stabilized zirconia, the electrolyte of solid-oxide fuel cells and oxygen sensors. The more oxygen vacancies you can create and keep mobile, the higher the ionic conductivity — which is exactly why zirconia is doped.
Doping zirconia (ZrO2) with about 8 mol% yttria (Y2O3) replaces some Zr4+ with Y3+; to keep charge balanced the crystal makes one oxygen vacancy for every two yttrium ions, filling the lattice with V_O•• that carry current at 800 to 1000 degrees C.
A vacancy in an ionic crystal always carries an effective charge, so it can never appear alone — its charge must be balanced somewhere.
Do not confuse the empty geometric space with the charged bookkeeping symbol. A vacancy contains zero atoms but, relative to the perfect lattice, carries a definite effective charge that must be tracked in every defect reaction.