an electronic defect
Not every defect is a misplaced atom — some are misplaced CHARGE. An electronic defect is a free electron, or its absence (a hole), roaming the crystal's electronic structure. Picture the electrons as students filling the seats of a lecture hall from the front: a full lower level (the valence band) and an empty upper level (the conduction band) separated by an empty gap of forbidden energies. An electron kicked up into the empty upper level is a free ELECTRON (e', it can carry current); the empty seat it left behind in the lower level is a HOLE (h•), which also carries current, behaving like a mobile positive charge.
Electronic defects appear three ways. Intrinsically, thermal energy lifts an electron across the band gap, nil ⇌ e' + h•, with n times p = K_i fixed by mass action and the gap size (a small gap means many carriers — this is why a wide-gap oxide like alumina is an insulator while a narrower-gap oxide can be a semiconductor). Extrinsically, an aliovalent DONOR dopant releases an electron and an ACCEPTOR captures one to make a hole. And through NONSTOICHIOMETRY, reducing an oxide creates electrons (O_O^x --> 1/2 O2 + V_O•• + 2 e', n-type) while oxidizing a metal-deficient oxide creates holes (p-type). In many ionic ceramics the carrier is not delocalized but hops as a localized POLARON.
Electronic defects are the dividing line between an insulating ceramic and a functional electroceramic. Their concentration, tuned by doping and firing atmosphere, decides whether an oxide is a capacitor dielectric (few carriers), a semiconductor thermistor or varistor, or a mixed ionic-electronic conductor. A key honesty: in an oxide, ionic and electronic defects are coupled through the oxygen exchange reaction, so you cannot change one without the other — which is exactly why the Brouwer diagram plots them together against oxygen pressure.
Barium titanate is a superb insulator when stoichiometric, but firing it in a low-oxygen atmosphere pulls out oxygen and injects electrons (V_O•• + 2 e'), turning it grey and semiconducting. Capacitor makers must therefore re-oxidize their fired multilayer capacitors to mop up those electronic defects and restore the insulation.
Electronic defects, unlike atomic ones, can be created or removed just by changing the oxygen atmosphere at temperature.
A hole is not a real particle but a useful fiction — the absence of an electron in an otherwise full band, which moves and carries current as if it were a positive charge. Treating it as a genuine defect is standard and works, but remember what it stands for.