Question

What is edge effect?

Answer 1

My guess is you're referring to the edge effect on electric fields. In many textbooks, and many problems dealing with electric fields you will see assumptions used to minimize the effect an edge has on the electric field you're attempting to analyze, which is done to simplify the problem. One common example is determining the electric field strength at a point a given distance from a conductor carrying X amount of current. It's often assumed that this is an infinitely long conductor. This removes the need to analyze the electric field due to the corner of the conductor, where the shape, sharpness of the corner, size, type of conductor, and amount of current can impact the actual effect that corner has on the electric field strength at a given distance from the conductor. Usually the impact of the corner is minor, so it can be ignored by conductor. assuming an infinite conductor.Other examples:-calculating the electric field X distance from a charged plate (assume the plate is infinite) 

Answer 2

edge effects are the assumptions used to minimize the effect of an edge in an electric field.