on-base percentage

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A batter's number of times of reaching base safely, excluding instances resulting from a fielder's choice or an error, divided by the total number of times batting minus sacrifices, expressed as a three-digit decimal and used as a measure of a batter's overall effectiveness.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun baseball The frequency with which a hitter successfully reaches first base.