ファースト (fāsuto) means “first base”

More examples

打球をファーストへ投げます。da kyū o fā su to e na ge ma su

I throw the batted ball to first base.

In baseball, it centers on the first-base area and the player stationed there to receive throws and make outs.

一塁ichirui: The first base that runners reach after leaving home plate.
一塁手ichiruishu: The defensive player assigned to guard the first base.

野球で、一塁の位置、またはそこを守る選手。ya kyū de ichi rui no i chi ma ta wa so ko o mamo ru sen shu

ファースト (fāsuto) is borrowed from English first as used in first base, with the same Japanese term also referring to the player at that position.

The English terms first base and first baseman distinguish the place from the player, but Japanese ファースト (fāsuto) can cover both meanings.