A Wigner-Seitz cell, in solid state physics, is a primitive lattice cell.
The cell may be chosen by first picking a lattice point. Then, lines are drawn to all nearby (closest) lattice points. At the midpoint of each line, another line is drawn normal to each of the first set of lines.
In the case of a three-dimensional lattice, a perpendicular plane is drawn at the midpoint of the lines between the lattice points. By using this method, the smallest area (or volume) is enclosed in this way and is called the Wigner-Seitz primitive cell. All area (or space) within the lattice will be filled by this type of primitive cells and will leave no gaps.