noun
infielder; baseman
Baseball term for a defensive player positioned in the infield. Covers positions such as first baseman, second baseman, shortstop, and third baseman.
彼は高校時代、内野手として活躍した。
He played as an infielder in high school.
内野手は素早い送球が求められる
Infielders need quick throws.
野手 is a general term for a fielder (both infielder and outfielder), while 内野手 specifically refers to an infielder.
Compound of 内野 (infield) + 手 (player). The reading ないやしゅ follows standard on'yomi.