Refers to a person who leads or commands a group, organization, or military unit. Slightly formal; often used in contexts emphasizing authority and the ability to unify people.
As the team leader, he brought everyone together.
That general was an excellent commander.
Compound of 統率 (tōsotsu, 'command; leadership') + 者 (sha, 'person'). The exact historical derivation is uncertain, but the word follows a common pattern for forming agent nouns.