From HandWiki

The range of a particle of given energy in an absorbing material is the average thickness of material ( usually defined parallel to its original direction) which it traverses before coming to rest. For a charged particle other than an electron, this is dominated by energy loss due to ionization; for electrons, the energy loss due to bremsstrahlung is more relevant. Curves and tabulations of the range due to ionization loss exist for different particles in different absorbers (see Barnett96, Serre67, Richard71). Fluctuations in the energy loss result in a distribution of range values for particles with the same energy, which is called range straggling.