Games: What players of the game engage in: Behavior according to certain rules, that characterize the game, that serves some purpose, like amusement, instruction, learning or deceit.
The term "games" is used for many different activities, and in various more specific senses. It is difficult to define it in such a way that the definition accounts for all usages. However, it is taken as more specific than the similar term play, in that a game is a game, and in particular that game, because of its satisfying certain rules, that one has to know in order to be able to play the game.
There is a mathematical theory of games, that relates to games like chess, bridge and poker, and more broadly to strategic behavior in economics and politics, that involves reasoning about possibilities, probabilities and pay-offs, and strategies with which one can optimize one's winnings or minimize one's losses.
There is also a tragic site to games: Very many of the games people play are played while believing them to be something other than games. Indeed, the common social roles - father, mother, boss, clerk, manager, leader, follower, doctor, nurse and so on - tend to be mostly games that the actors play while believing or pretending that the way they play it and think of it is the right way to act out and believe in such a role.