what do you call it ?

[Sam the Cat]

The small 1/2 circle thing that mounts inside a cabinet to keep an inset door from swinging too far in the cabinet ?

TIA

[X]

door stop..(but they do not have to be semicircles

[charlie b]

If it goes into the end grain end of the door it's a "bullet catch". There's typically a corresponding brass dimpled "thing" in the end grain of the door, the bullet catch is in the cabinet carcass/carcase.

charlie b

[Silvan]

It's a flungledootchie.