mod·er·a·tor 
(mŏdə-rā′tər)
(mŏdə-rā′tər)n.
1. One that moderates, as:
a. One that arbitrates or mediates.
b. One who presides over a meeting, forum, or debate.
2. The officer who presides over a synod or general assembly of the Presbyterian Church.
3. Physics A substance, such as water or graphite, used in a nuclear reactor to decrease the speed of fast neutrons and increase the likelihood of fission.