The Collaborative International Dictionary of English v.0.48:

Slugging average \Slug"ging av`er*age\ (Baseball)
   a measure of the effectiveness of a batter at reaching base
   and advancing other runners, calculated as the sum of the
   number of bases reached on each hit, divided by the total
   number of times at bat. A double counts two bases, a triple
   three, a home run four. Thus a batter with four singles, two
   doubles and a triple in 20 official times at bat would have a
   slugging average of 0.550, and a batting average of 0.350.
   Also called slugging percentage. Compare batting average.
   [PJC]