A pitcher receives a loss when a run that is charged to him proves to be the go-ahead run in the game, giving the opposing team a lead it never gives up. Losses are almost always paired with wins when used to evaluate a pitcher, creating a separate pitching term known as win-loss record.
Win-loss record took on a greater importance in the past for a different reason. In the time when pitchers routinely pitched complete games, bullpens were rarely at fault for losses. But today's specialization of relief pitchers has led to starters pitching fewer innings.
A starting pitcher does not necessarily receive a loss every time his team loses -- even if he exits the game with his team trailing. In such instances, if his team ties the game or takes the lead before eventually losing, it will be the pitcher who put the go-ahead run on base who takes the loss.
In A Call
"took the L," "losing pitcher"