foci
SayFOH-syUS/ˈfoʊ.kaɪ/UK/ˈfəʊ.kaɪ/
- 1Plural of focus.
An ellipse is defined by two foci and the sum of distances to them.
Researchers identified several foci of infection within the affected tissue.
Note
A Latin plural that only the countable meanings of focus can take: points where light rays converge, points of a conic, and things that attention or activity centers on. Focus as concentration, or as a photograph's sharpness, has no plural, so a picture is in sharp focus, never in sharp foci. Mathematical, optical and medical writing keeps foci, and the regular focuses is generally avoided there.
foci is a form of focus
focus, noun: The thing toward which activity, attention, or interest is mainly directed. Full entry
- Spelling
- focus → foci: irregular plural (no -s)
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