Thin radiolucent line which follows the outline of tooth is known as:

Because the PDL is composed primarily of collagen, it appears as a radiolucent space between the tooth root and the lamina dura. This space begins at the alveolar crest, extends around the portions of the tooth roots within the alveolus, and returns to the alveolar crest on the opposite side of the tooth. White and Pharoah, pg-134