lunule

noun2

1. •
   Anything crescent-shaped; a crescent-shaped part or mark; a lunula or lune.

   • A healthy lunule is usually most prominent on the thumb and may be invisible on the pinky finger.
   • The architect incorporated a lunule design into the arched windows of the cathedral.
2. •
   An area in front of the beak of many bivalve shells, sometimes shaped like a double crescent, but more often heart-shaped.