This phrase is used in baseball to describe the two pitchers assigned the win and loss for the game. These two pitchers are the ones who receive the decision once the game is over. In order to record a decision, the pitcher must pitch at least five full innings. There are always two pitchers of record for every game. If a relief pitcher is used, they may be known as the third pitcher of record, but this is unofficial and not the strict definition of the term.
The pitcher of record is used to assign a win to the winning pitcher, while assigning a loss to the opponent. If the phrase is used during the game, it is often applied to the pitcher most likely to become the pitcher of record once the game ends. One can easily calculate how many times a pitcher is listed as the pitcher of record by adding up all their wins and losses. Saves and missed saves do not qualify for a decision.