a point defect
A perfect crystal is like a vast, perfectly stacked warehouse where every atom sits on its assigned shelf in an endless repeating pattern. A point defect is a single mistake in that stacking at one spot — one shelf left empty, or an extra box wedged into the gap between shelves, or the wrong box sitting on a shelf. It is 'zero-dimensional': the disruption is localized to essentially one lattice site and its immediate neighbours, unlike a line defect (a dislocation) or a plane (a grain boundary).
The main families are the vacancy (a normally-occupied site left empty), the interstitial (an atom squeezed into a site that should be empty, the gap between the regular atoms), and the substitutional (a foreign atom sitting on a host site). Crucially, defects are not just accidents of a sloppy furnace: above absolute zero a crystal WANTS a certain number of them, because scattering a few vacancies through the lattice raises its entropy (disorder), and that lowering of the free energy outweighs the energy cost of making them. The equilibrium fraction follows a Boltzmann law, n/N is about exp(-E_f / kT), where E_f is the energy to form one defect, k is Boltzmann's constant and T the absolute temperature — so defect populations rise steeply with temperature.
In a ceramic these tiny gaps are the whole reason atoms can move at all: an ion can only shuffle through a dense solid by hopping into a neighbouring vacancy or squeezing between sites, so point defects are the vehicles of diffusion, of sintering, and of ionic conduction in solid electrolytes. A key honesty: a truly 'perfect' crystal exists only at 0 K; every real fired ceramic carries a built-in, temperature-set defect population, and the whole subject of defect chemistry is the bookkeeping of how many of each kind there are and what charge they carry.
In magnesium oxide (MgO) held at 1600 degrees C for firing, a small fraction of the Mg2+ and O2- sites sit empty as thermally generated Schottky vacancies; on cooling, many are frozen in. Those quenched-in vacancies are the paths along which magnesium and oxygen ions diffuse when the powder sinters into a dense part.
Point defects are not damage to be avoided but the working machinery of every high-temperature ceramic process.
A common misconception is that defects are always bad. In ceramics the opposite is often true: engineers deliberately introduce point defects (by doping) to switch on oxygen-ion conduction, colour, or capacitance.