An Extreme Programming Practice is a method or behavior used as part of a wider process in AgileSoftwareDevelopment, or in this case ExtremeProgramming.
ExtremeProgramming Practice - definition of Extreme: elaborating a practice which systematically excludes certain methods and practices:
"Extreme" means these practices get "turned up" to a much higher "volume" than on traditional projects.
XP ignores any other practice (like BigDesignUpFront) that does not appear on the list. The result is stable, productive, and very rapid because the practices support each other the more they are used together without interference.
An Extreme project is typically so stable and sedate it can lead to FortyHourWeeks without any schedule slips.
See ExtremeProgrammingPractices.