late 14c., "ruler, governor," 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 umpire" is from 1560s. Fem. form moderatrix attested from 1530s.
