League average at the position, or league average period?
League-average first, and then there's an overall adjustment based on position is my understanding. Meaning you don't get double credit for hitting well because of your position.
Here's the definition from FanGraphs:
"Offensive players – Take wRAA, UBR, and UZR (which express offensive, base running, and defensive value in runs above average) and add them together. Add in a positional adjustment, since some positions are tougher to play than others, and then convert the numbers so that they’re not based on league average, but on replacement level (which is the value a team would lose if they had to replace that player with a “replacement” player – a minor leaguer or someone from the waiver wire). Convert the run value to wins (10 runs = 1 win) and voila, finished!"
