A measure of the ability of matter to reduce the kinetic energy of a particle passing through it. The linear stopping power, −dE/dx, is energy loss of a particle per unit distance. The mass stopping power, (1/ρ)dE/dx, is the linear stopping power divided by the density (ρ) of the substance. The atomic stopping power, (1/n)dE/dx = (A/ρN)dE/dx, is the energy loss per atom per unit area perpendicular to the particle's motion, i.e. n is the number of atoms in unit volume of the substance, N is the Avogadro number, and A is the relative atomic mass of the substance. The relative stopping power is the ratio of the stopping power of a substance to that of a standard substance, usually aluminium, oxygen, or air.