Final answer:
Self-interstitial defects are a type of point defect in crystalline solids, occurring when an atom or ion is added to an interstitial site.
Step-by-step explanation:
The self-interstitial defect belongs to the category of point defects in crystal solids. Point defects are various types of irregularities in the regular atomic or ionic arrangement in a crystalline solid. While vacancies are empty spaces where an atom or ion should be, self-interstitial defects occur when an atom or ion is squeezed into an interstitial site, which is a small space between the regular atomic positions. This type of defect can create distortions in the lattice because the extra atom or ion may not fit neatly within the structure. It is distinguished from interstitial impurities, which involve impurity atoms occupying these interstitial sites.