a point defect
Picture a perfect crystal as endless wallpaper: the same little cluster of atoms stamped again and again, in perfect lock-step, in every direction. Now imagine one stamp on that wallpaper is wrong — a single dot is missing, or an extra dot has been squeezed in, or one dot is the wrong colour. That tiny, localised mistake, no bigger than a single atom, is a point defect. It is the very first place where the crystal's perfect periodicity breaks down, and it is the smallest kind of imperfection there is.
Point defects are called zero-dimensional because the disturbance is confined to essentially one atomic site (it has no length, area, or volume that runs through the crystal — contrast a dislocation, which is a defect strung out along a LINE, or a grain boundary, which is a defect spread over a SURFACE). There are only a few basic kinds. A site can be empty when it should hold an atom (a vacancy). A host atom can be jammed into a gap between the normal sites (a self-interstitial). Or a foreign atom can sit in the crystal, either replacing a host atom on its site (a substitutional impurity) or wedged into a gap (an interstitial impurity). Every zero-dimensional imperfection is one of these, or a small combination of them.
Here is the fact that surprises most beginners: point defects are not rare accidents you could polish away — every real crystal above absolute zero MUST contain them. They actually lower the crystal's free energy by adding disorder (entropy), so nature insists on a certain population of them at any temperature above zero. That is why they matter so much: this unavoidable sprinkling of missing and misplaced atoms is what lets atoms move through solids (diffusion), lets ionic crystals conduct electricity, gives some crystals their colour, and sets how strong, how pure, and how reactive a real material actually is.
In a bar of pure copper at room temperature, most of the crystal is flawless — but scattered through it are empty atom sites (vacancies) and, if the copper is not perfectly pure, foreign atoms sitting on copper sites (substitutional impurities). Each is a single-atom disturbance, a point defect, invisible to the eye yet decisive for how the copper conducts and how easily its atoms rearrange when heated.
A point defect is a single-atom-sized flaw: a missing, extra, or foreign atom breaking the perfect pattern at one site.
Do not think of point defects as defects that could, in principle, all be removed. Above absolute zero a certain population is thermodynamically required — a truly defect-free crystal at finite temperature would violate the second law of thermodynamics.