focus

[17] Latin focus meant ‘fireplace’, and in post-classical times it came to be used for ‘fire’ itself – hence French feu, Italian fuoco, Spanish fuego, all meaning ‘fire’, and hence too the English derivatives fuel and fusillade. The first writer known to have used it in its modern sense ‘point of convergence’ was the German astronomer Johannes Kepler, in 1604, but the reason for his choice of word is not clear. It may have been some metaphorical notion of the ‘hearth’ symbolizing the ‘centre of the home’, but it has also been suggested that it may have been preceded and inspired by the use of focus for the ‘burning point’ of a mirror (not actually recorded until somewhat later). The philosopher Thomas Hobbes appears to have introduced the term into English, in 1656. A medieval Latin derivative of focus was focārius, from which French got foyer ‘hearth, home’, borrowed by English in the 19th century for a public entrance hall or lobby. => FOYER, FUEL, FUSILLADE

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   The origin of the word is in the identical Latin word meaning 'fireplace,' 'hearth,' as this was the central or focal point in a Roman house. In popular Latin focus later became the word for 'fire' itself. Hence French feu, Spanish fuego, Italian fuoco, as the standard word for 'fire.'