Superstition refers to any irrational action by a participant in a baseball game that is based on the belief that this will somehow have a positive effect on the result of the game. The action is only a superstition if the relationship between the action and its intended result is so tenuous that it cannot be empirically measured.
Baseball players are known to be a superstitious lot. Typical practices include eating only certain foods before a game, wearing a particular item of clothing, following a particular routine when they go to their position or come to bat, or not altering anything when they are on a hot streak (for example, not washing a batting helmet while on a hitting streak, thereby letting it accumulate a thick layer of dirt and grime). Superstitions are a common area of study by cultural anthropologists as they serve a purpose of deflecting stress or blame for failure, something that is very much needed in baseball where repeated failure is the common lot of all players.
Fans also are superstitious. One common practice is not to mention aloud an on-going no-hitter in order not to "jinx" the player, or believing that a team's lack of success is based on a curse (e.g. the Curse of the Bambino or the Curse of the Billy Goat) and not on the failings of the players or management.
- Associated Press: "Baseball superstitions abound for players and managers alike", USA Today Sports, September 1, 2017. 
- George Gmelch: "Baseball Magic", Human Nature, Vol. 1, Nr. 8, 1978, pp. 32-40. 
- George Gmelch: "Superstition and Ritual in American Baseball", Elysian Fields Quarterly, Vol. 11, No. 3, 1992, pp. 25-36.