focus [M17th] In Latin focus meant ‘hearth, fireplace’, the centre of the household. It was first used in 1604 in Latin in the geometrical sense by the German astronomer Johannes Kepler (1571–1630), possibly with the idea that the focus of a conic section was like a fire as a source of light, a sense the word was being used for in English by 1656. The optical sense was in use a few years later, with focal point appearing before the end of the century. The focus groups of modern marketing are much more recent, dating from the early 1930s. Latin focus became foyer in French, in the sense ‘hearth’. It came to be used in French for the ‘green room’ in a theatre, and then for the area that the public could gather in during intervals, a sense first found in English in 1859. See also fuel.