n (physics) a substance (often water or graphite) used to decrease the speed of fast neutrons in a nuclear reactor and hence increase likelihood of fission
n a device used to deaden some of the noise from a firearm, although not to the same extent as a suppressor or silencer.
n (Ireland) At the University of Dublin, either the first (senior) or second (junior) in rank in an examination for the degree of Bachelor of Arts.