Plural of 'focus'; multiple central points of concentration, activity, or attention.
Direct plural form of Latin 'focus' (hearth/fireplace). Most English plurals add '-es' or '-s', but some Latin and Greek words keep their original plurals: focus/foci, like alumnus/alumni.
The plural 'foci' is one of English's gifts from centuries of scientific Latin—while we usually add '-s', we still use the original Latin plural when talking about multiple points in ellipses and parabolas, honoring mathematical tradition.
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