In electrical engineering and electromagnetism, epsilon (represented by the Greek letter ϵ) denotes the permittivity of a dielectric medium, which measures how an electric field affects and is affected by that specific insulating material. It quantifies the ability of a substance to store electrical potential energy within an electric field. Permittivity is typically expressed relative to the permittivity of a vacuum (free space) through a dimensionless multiplier known as relative permittivity or dielectric constant, playing a vital role in calculating capacitance, electromagnetic wave propagation speeds, and electrostatic forces within electronic components and transmission lines.