Foci

Part of speech: noun

Pronunciation: /ˈfəʊ.kaɪ/

Definitions

1. A central point of focus within a geometric figure, such as an ellipse or hyperbola
2. the plural form of the term referring to points that influence or direct attention, feelings, or thoughts
3. a hub or nucleus of activity or interest that gathers around a particular theme or subject matter

Etymology: Foci is the plural of "focus," and the surprise lies in where the original word came from. The Latin "focus" meant "hearth" or "fireplace," the place where a household's fire was kept. It is related to "fuel," which makes sense: the hearth was the burning centre of the home. The move from fireplace to geometry came from Johannes Kepler, who used the term while describing the elliptical orbits of planets. An ellipse has two special points, and Kepler saw that they behaved like a hearth. A light placed at one of them would have all its light reflected off the curve and gathered at the other, so he named these points after the fire itself. From that mathematical sense, the word spread into optics and the sciences, and eventually into everyday English, where a focus now means a centre of attention or interest. The plural "foci" is the classical form, used mainly in technical contexts, while "focuses" is common in general writing.

Synonyms: centers, nuclei