Half a move??

Here's a small puzzle my friend gave me but question is: Checkmate in 0.5 move??

First to solve gets a free trophy of their choice.

Even the daily puzzles look hard compared to this.

Kh2#

Free trophy? I have never needed to pay for trophy on chess.com :)

Finish castling checkmate so, -0# or Rf1#

If that is the trick in this puzzle, then the puzzle information is wrong:

In two-player sequential games, a ply refers to one turn taken by one of the players. The word is used to clarify what is meant when one might otherwise say "turn".

"Turn" is problematic since it means different things in different traditions. For example, in standard chess terminology, one move consists of a turn by each player; therefore a ply in chess is a half-move. Thus, after 20 moves in a chess game, 40 plies have been completed—20 by white and 20 by black. 

It would have been "somehow" correct if it said 0.5 ply though.

However, "Mate in one move" can mean either 1 or 2 plies. It's as correct as a joke problem can be.

If the joke is based on non-chess terminology, in other words, a changed definition of a move in chess, then yes.

We have a winner! Choose your trophy.

Lol?? It was just a joke

Yes, a joke based on self-made definitions.

nice but what's the trophey

First to solve gets a free trophy of their choice.

Super person or chess duel

Good lord! I taught philosophy and never engaged in splitting hairs this fine. It was a joke, an amusement, a diversion. And since there is only one move in all of chess that involves a player touching TWO OF HIS OWN PIECES to make a single move, I'm going to side with the OP and let him have his definition of "half a move."

Mate in one is always a one ply, because a player cannot mate himself.

No, the term 'move' is ambiguous. For '50-move rule', it means 100 ply. For 'mate in 2 moves' it usually means 3 ply. '1 move = 2 ply' is not always true.

Sometimes the terms 'full move' and 'single move' are used to denote two ply and one ply respectively.

In a retro, it may black to move and every move he has falls into a mate. 2 ply for a mate in one.

nobody says 'half-move,' 'half-move' is incorrect.  this is a 'mate in one'  i.e. 'one move'  BigDogg, 50 move rule refers to 50 moves on both sides.  they use that term to place emphasis on the 50th move for a player 'white or black' to claim a drawn position.  if they said '100 move rule,' it wouldn't reference a particular person's 50th move.

'half-move,' however is non-sensical.  it is mate in 1, not mate in .5 moves.  half-move would reference the opponent's move after he has already been checkmated, which obviously makes no sense.

not much of a problem...

I bet you have never seen one.