In mechanical, civil, and materials engineering, epsilon (ϵ) universally denotes strain, which is the dimensionless measure of the physical deformation or elongation of a material resulting from applied external stress (σ). It represents the ratio of total change in length to the original resting length (ΔL/L0). In electrical, electronic, and RF engineering, epsilon represents the permittivity of a dielectric medium, measuring its ability to store electrical potential energy in an electric field and determining capacitance values and signal propagation speeds in transmission lines.