base on balls

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

• noun An advance to first base that is awarded to a batter who takes four pitches that are balls.

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

• noun baseball An instance where the batter is allowed to go directly to first base without the possibility of being put out, due to the opposing pitcher delivering four balls.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (baseball) an advance to first base by a batter who receives four balls