Basically, the fraction of a player's plate appeareances that resulted in his reaching base safely. It is defined officially as (H + BB + HBP) / (AB + BB + HBP + SF) 1-OBP will tell you approximately what fraction of the times a player has gone up to the plate have resulted in outs. It was created by Branch Rickey in the early 1950's in its present form, though it had ninteenth-century predecessors, such as "Reached First Base". Adopted as an official stat in 1984. Sometimes (more correctly) known as On Base Average.
In baseball statistics, on base percentage (OBP) (sometimes referred to as on base average [OBA], as the statistic is rarely presented as a true percentage) is a measure of how often a batter reaches base for any reason other than a fielding error, fielder's choice, fielder's obstruction, or catcher's interference (the latter two are ignored as either times-on-base [TOB] or plate appearances in calculating OBP). OBP is added to slugging average to determine on-base plus slugging (OPS).