Latin for 'hearth' — Kepler adopted it in 1604 for where a lens concentrates light, because focused light produces fire.
Definition
The point at which rays of light or other radiation converge or from which they appear to diverge. The centre of interest or activity. Clear visual definition. The state of concentrated attention.
The Full Story
Latin17th centurywell-attested
From Latin 'focus' (hearth, fireplace, thedomesticfire at the centre of a Roman home). The word had nothing optical or mental about it in classical Latin — it named the physical hearth around which Roman family lifewas organised. The household hearth was the altar of the Lares and Penates (household gods), simultaneously a functional
Did you know?
TheEnglish word 'curfew' comes from Anglo-French 'coeverfu,' from Old French 'couvre-feu' (cover the fire), from 'couvrir' (to cover) + 'feu' (fire, from Latin 'focus'). Medieval regulationsrequired fires to be banked or covered at a certain hour to prevent conflagrations. The connection to 'focus' is hidden but direct: both 'curfew' and
fire just as the domestic hearth does. Kepler's coinage spread rapidly through 17th-century science. The extended sense of 'focus' as the centre of mental attention developed in the 18th century, following the same logic: to focus attention is to gather it to a single burning point as a lens gathers light. The PIE root underlying 'focus' is uncertain; connections to *bʰōwos (burning) or *dʰewbʰ- (smoke, haze) have been proposed but the pre-Latin etymology remains disputed. 'Foyer' (the hearth-area, then the entrance hall) and French 'feu' (fire) descend from the same Latin word. Key roots: focus (Latin: "hearth, fireplace").