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The threshold displacement energy T d {\displaystyle T_{d}} is the minimum kinetic energythat an atom in a solid needs to be permanentlydisplaced from its lattice site to a defect position.It is also known as "displacement threshold energy" or just "displacement energy".In a crystal, a separate threshold displacementenergy exists for each crystallographic direction. Then one should distinguish between theminimum T d , m i n {\displaystyle T_{d,min}} and average T d , a v e {\displaystyle T_{d,ave}} over alllattice directions threshold displacement energies.In amorphous solids it may be possible to define an effectivedisplacement energy to describe some other average quantity of interest.Threshold displacement energies in typical solids areof the order of 10 - 50 eV.