When electrons are trapped into the crystal in place of an anionic vacancy, the defect is known as ____ ?

Electrons are trapped inside an crystal in pace of an anion in case of metal excess defect. This is a type of non-stoichiometric defect. In order to maintain charge neutrality, electrons occupy the vacancies of anions. These lattice points where electrons are present imparts colour to the crystal and are often referred to as Farbene centres or F-centres.