Complex sentence: However, earned runs are specially denoted because of their use in calculating a pitcher's earned run average (ERA), the number of earned runs allowed by the pitcher per nine innings pitched (i.e., averaged over a regulation game).
Split into simpler sentences: However, earned runs are specially denoted because of their use in calculating a pitcher's earned run average (ERA). One of the most prominent of baseball's traditional statistics for pitchers, the ERA is defined as the number of earned runs allowed per 9 innings pitched (i.e., averaged over a regulation game).