Nature, or Reality, is one of the two main players in various games of prediction (in particular, in conformal prediction and competitive on-line prediction).

The traditional connotation of "Nature", going back centuries, is that it is neutral. von Neumann and Morgenstern might not have used "Nature" in their book, but "games against Nature" were discussed soon thereafter by game theorists with the assumption that Nature is neutral; Milnor (1951) for example says Nature's payoff is "unknown or identically zero" and that "there is no reason to believe that nature is against us". Accordingly, the term "Reality" is usually used in game-theoretic probability (e.g., in Shafer and Vovk, 2001), where reality is not considered to be neutral.