| an extraordinary or unusual thing, person, or event; an exceptional example or instance. |
| an arrangement of five objects, as trees, in a square or rectangle, one at each corner and one in the middle. |
| moderator (mŏd'ə-rā'tər) Pronunciation Key
A substance, such as graphite, water, or heavy water, placed in a nuclear reactor to slow neutrons down to speeds at which they are more likely to be captured by fissionable components of a fuel (such as uranium-235) and less likely to be absorbed by nonfissionable components of a fuel (such as uranium-238). Also called neutron moderator. See also slow neutron. |