The mass that an electron in a crystal acts as if it had. The value of an effective mass can be much smaller or larger than the mass of a free electron and can even be negative. The effective mass can also be anisotropic. The concept of effective mass is very useful in the quantum theory of solids. For example, many impurity states can be regarded as being like the states of a hydrogen atom modified by the dielectric constant of the solid and the effective mass of the electron in the solid.