Electric flux $Phi_E$ = integral(E . dA) = integral(Ecos(theta)dA) where theta is the angle between E and the outward normal to the surface element. SI unit: N $m^2$/C (or V m). For a uniform field through a flat surface: Phi = EAcos(theta). Flux is positive when field lines exit the surface, negative when they enter. For a closed surface, outward normal is always used. Flux through a closed surface is zero if no net charge is enclosed (as many lines enter as leave). Important: if a charge Q is at a corner of a cube, only Q/8 is "enclosed" by that cube (imagine 8 cubes sharing the corner), so flux through that cube = $\frac{Q}{8*epsilon_0}$.