Definition of focal axis in English:

focal axis

noun

  • 1Geometry the line passing through the focus or foci of a conic section, perpendicular to the directrix.

  • 2Optics the line which passes through the focal point (or points) of a curved mirror or lens and its centre; the line which passes through the centre of each lens in a compound lens, telescope, microscope, or other optical instrument.

Origin

Late 18th century.