A periapical radiograph shows a 'tooth within a tooth' radiographic appearance. What developmental anomaly does this represent?

Dens invaginatus results from an infolding of enamel and dentin into the tooth before calcification, producing the classic 'tooth within a tooth' appearance — most commonly affecting the maxillary lateral incisor, and clinically significant because the invagination creates a pathway for early pulpal infection even through an apparently intact crown.