Radiolucency marked by arrow is called as:

Diffuse radiolucent areas with ill-defined borders may be apparent radiographically on the mesial or distal aspects of teeth in the cervical regions between the edge of the enamel cap and the crest of the alveolar ridge. This phenomenon, called  cervical burnout,  is caused by the normal configuration of the affected teeth, which results in decreased x-ray absorption in the areas in question. Close inspection reveals intact edges of the proximal surfaces. The perception of these radiolucent areas results from the contrast with the adjacent, relatively opaque enamel and alveolar bone. Such radiolucencies should be anticipated in almost all teeth and should not be confused with root surface caries, which frequently have a similar appearance. White and Pharoah, pg-131