Point which is defined by the intersection of the optical axis and the backwards extrapolated path of a ray that (1) is tending towards infinity, (2) was parallel to the optical axis in the incident beam, and (3) that was deflected by an electron lens.

The here given definition applies to electromagnetic lenses and is a more general definition compared to those typically given in classical optics. The focal point is an inherent property of a lens. Definition only holds if the (virtual) source is "at infinity", meaning very far away as compared to the focal length of the objective lens. This is for an ideal lens, in the reality there might be aberrations affecting the beam profile at the focal point.