moderator (n.)

late 14c., moderatour, "that which regulates the movement of the celestial spheres," from Latin moderator "manager, ruler, director," literally "he who moderates," from moderatus "within bounds, observing moderation;" figuratively "modest, restrained," past participle of moderari "to regulate, mitigate, restrain, temper, set a measure, keep (something) within measure," from PIE root *med- "take appropriate measures."

Meaning "one who acts as an arbitrator, person who presides at a meeting or disputation" is from 1560s. Fem. form moderatrix attested from 1530s.