An Arrow.

Arrows were added in Delver Alpha 8-13-12. They are the ammunition for Bows, without arrows you cannot use the bow. They are found in stacks of 20.  When you fire an arrow and it damages an enemy, the arrow fired is consumed. While when you fire an arrow and it lands on the ground, it can be picked up and will be returned to the stack it came from. In the next update the stacking feature will be fixed.