In chess, and presumably other games where two players alternate turns, the word move has two meanings:

  • One player moving one of his pieces
  • White moving a piece, and Black's move in response

Presumably because the game can end with White making the final move (I can't think of any other reason this term would be necessary), the technical term for the first of these meanings is ply. Thus, a chess game in which White moves twenty one times and Black moves twenty times is said to comprise forty one plies.