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.
Definition of focal axis in US 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.