Recessed Crown
Also: Target Crown
A crown machined below the muzzle face so the delicate rifling edge sits protected in a recess where it is shielded from dings and wear.
A recessed crown sets the actual rifling edge down inside a small counterbored pocket rather than flush with the end of the muzzle. The crown is the last surface the bullet and escaping gas touch, and any nick or unevenness there can disturb the bullet and open up groups. Sinking it into a recess shields that critical edge from contact.
Because the edge sits below the muzzle face, the recess protects it from the kinds of bumps that happen during cleaning, transport, or a careless rest on hard ground, all of which could otherwise damage the bore exit. This is why a target crown, sometimes called a recessed crown, is a common choice on precision rifles where protecting that surface is worth the extra machining on the barrel.