center field

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

• noun The middle third of the outfield, behind second base.
• noun The position played by the center fielder.

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

• noun baseball The part of a baseball field which is beyond the infield and straight ahead left if you stand on home plate and face the pitcher.
• noun baseball The defensive position in the outfield in the middle, typically played by a player that can run fast.
• noun idiomatic A central role in some activity that requires speed.