Complex sentence: Von Neumann and Morgenstern (1944) defined a "fictitious player" as a player with only one strategy, added to an n-player game to turn it into a n +1-player zero-sum game.
Split into simpler sentences: Von Neumann and Morgenstern (1944) defined a "fictitious player". This was a player with only one strategy, added to an n-player game to turn it into a n +1-player zero-sum game.