What is the plural form of focus?

The plural form of "focus" is foci, or the English plural focuses.

Latin words in which the singular ends in -us are typically pluralized by dropping the -us and adding an "i".

Examples include "stimuli" for stimulus, and "rhombi" for "rhombus."

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Foci and focuses are both acceptable.

(When used as a verb, "focuses" is the only acceptable form for the third person singular.)

"Foci" is formed using the Latin rule for plurals. It may seem more erudite in written communication, but in spoken form (foe-sigh) most people will not recognize the word. "Focuses" is a bit more awkward because of the multiple "s" sounds, but the general population is more likely to recognize it. If you are doing professional or scholastic writing, you should refer to your organization's style manual for the preferred form.