Counter Sunk Bolt:

A counter sunk bolt is a type of fastening that sits flush with the surface of the material it occupies. These screws typically are used so that you can cover them easily with either a screw cap or piece of wood.  The typical angle of a countersunk screw head is 82 degrees. But this varies depending on the application. Verb (used with object), coun·ter·sank, coun·ter·sunk, coun·ter·sink·ing. to enlarge the upper part of (a cavity). Especially by chamfering, to receive the cone-shaped head of a screw, bolt, etc. Cause (the head of a screw, bolt, etc.) to sink into a prepared depression so as to be flush with or below the surface.

General:

A countersunk Bolt is a screw that is “sunk” into a piece of lumber. The head will sit below the surface of the material and it can easily be filled with a wood plug or filler. Bolts and large hex-head screws can also be countersunk. Each require a special countersink bit for the best results. These screws typically are used so that you can cover them easily with either a screw cap or piece of wood. Other applications include the need for a fastening not to protrude. So other objects can rest upon it stably, or to fasten it further to another object.