An estimate of a hitter's weighted on-base average based on the quality of contact, stripping out the luck in where balls landed.
Expected wOBA assigns each batted ball an expected outcome from its exit velocity and launch angle (and sprint speed on certain plays), then rolls those up the same way wOBA weights real outcomes. The result is what a hitter 'should have' produced given how they hit the ball.
The gap between xwOBA and actual wOBA is a luck signal. A hitter outperforming their xwOBA has been getting favorable results on their contact; one underperforming it has been hitting into bad luck that often corrects.
The xwOBA-versus-wOBA gap is the basis of the Lucky and Unlucky leaderboards. Unlucky hitters are candidates whose underlying contact supports more than their recent results show.
xwOBA estimates a hitter's weighted on-base average from the quality of their contact — exit velocity and launch angle — rather than from where the ball happened to land. It approximates deserved production.
A hitter with a wOBA higher than their xwOBA has gotten better results than their contact quality typically yields, which tends to regress. A wOBA below xwOBA suggests unlucky results that often improve.