One who, or that which, moderates, restrains, or pacifies.
- One that moderates, as:
- One that arbitrates or mediates.
- One who presides over a meeting, forum, or debate.
- The officer who presides over a synod or general assembly of the Presbyterian Church.
- Physics A substance, such as water or graphite, that is used in a nuclear reactor to decrease the speed of fast neutrons and increase the likelihood of fission.