noun
leader; prime mover; originator
Refers to a person who takes the lead in an initiative, movement, or project; emphasizes active guidance and driving force rather than just a figurehead.
彼はその改革運動の主導者だった。
He was the prime mover behind the reform movement.
プロジェクトの主導者として、彼女はチームをまとめた。
As the project leader, she brought the team together.
指導者 means 'instructor' or 'guide' who teaches or directs, whereas 主導者 emphasizes taking the initiative and leading from the front.
Compound of 主導 (shudō, 'leading', 'taking the initiative') + 者 (sha, 'person'). The word 主導 is itself a combination of 主 (shu, 'main') and 導 (dō, 'guide').