Drugs/substances taken by athletes that help improve their performance. Often illegal or, at least, banned by the governing body of the sport the athelete particpates in, these substances are taken for the specific reason to improve performance. Performance Enhancing Drugs - PEDs can be a reference to many different types of substances including anabolic steriods, human growth harmones, and certain stimulants.
PEDs have become a bane to statheads' existence because it's become impossible to quantify the effect its had on sports and the statistics accumlated. For example, the "PED era" in baseball is generally accepted to be from the late 1980s through (appx) 2010. This has put a general black cloud over such hallowed numbers like most home runs in a season (Barry Bonds - 73 in 2001), most career home runs (Barry Bonds - 762), and most Cy Young awards (Roger Clemens - 7). It even has affected those who never have been accuesed of PED use, but played in the PED era. In 2013, no player was elected into the baseball Hall of Fame, despite eligbile candidates like Barry Bonds, Mark McGwire, Craig Biggio, Roger Clemens.