The distance from a lens, lens system, or curved mirror to its focus along its focal axis.
The distance between any given point of an ellipse or hyperbola and one of its foci. In early use also: the distance between the foci of an ellipse or a hyperbola and its centre.
3The distance between any given point of a parabola and its focus. In early use also: the distance between the focus of a parabola and its vertex.
Late 17th century; earliest use found in William Molyneux (1656–1698), experimental philosopher and constitutional writer. From focal + distance.