The base itself or the position in baseball.
ランナーが一塁にいる。
There is a runner on first base.
Abbreviation of 一塁手. Used in baseball to refer to the player.
See also: 一塁手
彼は高校時代、一塁を守っていた。
He played first base in high school.
one fort
Rare, literal meaning. Not commonly used outside of historical or military contexts.
古文書に「一塁を築く」とある。
An old document says 'build one fort'.
The batter advanced to first base.
He plays first base on the team.
The runner was called out at first base.