1) A neutral subatomic particle having a mass of 1.0087 amu.
2) a particle of matter that has a mass of 1.009 amu but no electrical charge. These particles are constituents of the nucleus of all elements except for hydrogen (H). Neutrons can exist in a free state. Because neutrons have no electrical charge, free neutrons have great penetrating capabilities and are highly damaging to living tissue.
The number of neutrons an element contains can vary (see neutron variability), however, the average number of neutrons any particular element contains can be figured by subtracting the atomic number of an element from it's atomic mass average and rounding to the nearest whole number. When doing this, the resulting number should be noted something similar to: "On average this element contains X neutrons."
3) A heavy subatomic particle with zero charge; found in an atomic nucleus.