From The Collaborative International Dictionary of English v.0.48:
Hitter \Hit"ter\, n.
1. One who hits or strikes; as, a hard hitter.
[1913 Webster]
2. (Baseball) The batter; as, there are two strikes on the
hitter.
[PJC]
1. A baseball player who gets many extra-base hits.
2. A person who is very important or is very effective at his
profession.
[PJC]