Flux of an electric field
If in a plane surface area of ∆s, a uniform electric field E exists, and makes an angle θ with the normal to the surface area (positive normal - you can arbitrarily decide which direction is positive), the quantity
∆Φ = E ∆s cos θ
is called the flux of the electric field through the chosen surface.
If ∆s is represented as a vector
∆Φ = E.∆s
Where E and ∆s are vectors and ∆Φ is a scalar quantity.
(Chapter: Gauss's Law)