Effective mass

  When a particle interacts with surrounding forces, it gains some mass due to the interaction with the forces. This is called as effective mass. In the context of solid state physics, the electron due to the periodic potential seems to gain some mass and its behavior will be different from that in an vacuum.

Effective mass $m* =\dfrac{h^{2}}{\dfrac{d^{2}E}{dk^{2}}}$

E-K diagram for electron moving in a periodic potential