noun
young baseball player; high-school baseball player
Refers specifically to a teenager who plays baseball, most commonly a high-school baseball player. Often used in the context of the national high-school baseball tournament (甲子園).
彼は高校球児として甲子園を目指している。
As a high-school baseball player, he is aiming for Koshien.
球児たちは毎日厳しい練習に励んでいる。
The young baseball players train hard every day.
Compound of 球 (ball, baseball) and 児 (child, youth). The exact historical derivation is uncertain, but the term is conventionally used for young baseball players.