Effective mass (solid-state physics)

In solid state physics, a particle's effective mass (often denoted m ∗ {\textstyle m^{*}} ) is the mass that it seems to have when responding to forces, or the mass that it seems to have when interacting with other identical particles in a thermal distribution. One of the results from the band theory of solids is that the movement of particles in a periodic potential, over long distances larger than the lattice spacing, can be very different from their motion in a vacuum.

Source: Wikipedia — Effective mass (solid-state physics) (CC BY-SA 4.0)