noun
winning pitch; go-to pitch
Primarily used in baseball for a pitcher's best or decisive pitch, often thrown in a critical situation to get an out.
彼の決め球はフォークボールだ。
His go-to pitch is the forkball.
ピッチャーはここで決め球を投げてきた。
The pitcher threw his winning pitch here.
決め手 is a broader term for a decisive factor or finishing move in various contexts (sports, games, arguments), while 決め球 is specific to a pitcher's decisive pitch in baseball.
Compound of 決め (kime, 'deciding') + 球 (tama, 'ball'). The exact historical derivation is uncertain, but it is a natural sports term formation.