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, dropped/uncaught third strike, 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). It first became an official MLB statistic in 1984.
The on-base percentage of all batters faced by one pitcher or team is referred to as on-base against.(Wikipedia)