In my practice it would be used with a clenched fist and is just referred to as a backfist, although that would just be one of several variations of using the back of the fist to strike with. The way in which you have used it we would tend to do so from the leading hand, dropping the hand down then using the back fist strike in a very similar way as you have done. If you are striking with the wrist butt rather than the back of the hand then we would call that Cranes Head.
If you are striking with the wrist/neck of the forearm, we'd call is an "oxjaw". I've never thrown an oxjaw in that way though. If you are striking with the back of the hand/knuckles, I'd call it an (inverse) back fist.
Are you hitting with the first knuckle? If so, Sow Choi in Choy Li Fut. But we would have a bit more bend in the arm to protect the elbow.
We call it a "ridgehand" in Kajukenbo if you are striking with with kind of the thumb side of the hand. Think a "karate chop" instead of striking with the pinky section of your hand, you're swinging and hitting with the thumb side. If it's knuckles, we call it a roundhouse punch.
That is an overhand strike in boxing. See how your hand drops first It's not an "orthodox" boxing strike, but still very powerful. Please see below. There are two great videos in there. How To: Over-Hand Right
