Google is your friend.
"In two-player sequential games, a ply is one turn taken by one of the players. The word is used to clarify what is meant when one might otherwise say "turn". ... For example, in standard chess terminology, one move consists of a turn by each player; therefore a ply in chess is a half-move."
@brrrsaid Does anyone know what is meant by a "half move" ? π΅
In two-player sequential games, a ply is one turn taken by one of the players. ... 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.
Ply (game theory) - Wikipedia