Zeta potential refers to the electric potential at the boundary between the stationary layer of fluid attached to a particle's surface and the mobile fluid layer. It is important because various motions that affect colloid stability, like Brownian motion and sedimentation, are determined by zeta potential rather than surface potential. Zeta potential can be measured using electrophoresis - by applying an electric field to observe particle migration - or laser Doppler velocimetry. Measurements of zeta potential are used in applications like sewage treatment, paints, printing inks, and monitoring flocculation.