noun
first proponent; initiator and advocate
Refers to the person who first proposes and actively promotes an idea, theory, or movement.
彼はその改革の首唱者として知られている。
He is known as the first proponent of that reform.
この理論の首唱者は19世紀の科学者だ。
The initiator and advocate of this theory was a 19th-century scientist.
Compound of 首 (head, first) + 唱 (chant, advocate) + 者 (person). The exact historical derivation is uncertain, but the kanji composition clearly conveys the meaning of 'first advocate'.