1. A collection of plates or cylinders with electrostatic properties which cause an electron beam to refract in a manner similar to the refraction of light by an optical lens.
2. An arrangement of electrostatic fields that acts upon beams of charged particles similar to the way a glass lens acts on light beams.
3. An electrostatic field with axial (a line of symmetry of an optical system, such as the line passing through the center of a lens) or a plane symmetry which acts upon beams of charged particles of uniform velocity; such as, glass lenses act on light beams.

The action of electrostatic fields with axial symmetry is similar to that of spherical glass lenses, whereas the action of electrostatic fields with plane symmetry is like that of cylindrical glass lenses.