Refers to a person who publicly supports or proposes an idea, theory, or cause.
He is known as a proponent of educational reform.
その理論の提唱者はまだ若い研究者だ。
The advocate of that theory is still a young researcher.
提唱者 refers to someone who advocates or proposes an idea publicly, whereas 発起人 also implies taking concrete steps to get the project started.
Compound of 提唱 (advocacy, proposal) + 者 (person). 提唱 is from 提 (present, carry) and 唱 (chant, advocate).