Plural of “focus”

The plural of focus is foci (some dictionaries also accept focuses).

Why

"Focus" is Latin for "hearth" or "fireplace," the center of a Roman household, a sense Johannes Kepler extended metaphorically in the 17th century when he used "focus" for the fixed points defining an ellipse — the geometric idea that light or attention converges there the way a family gathered around a hearth. As a fourth-declension Latin noun, its classical plural is "foci," which English kept for the specifically mathematical, scientific, and medical senses of the word.

Outside those technical registers, though, "focuses" has become overwhelmingly dominant — when people talk about someone's various "focuses" in life, or a meeting covering several "focuses," the regularized plural is essentially the only form used, making this one of the starkest register splits among the Latin -us words on this list.

Usage note

Use "foci" specifically in geometry (an ellipse's two foci), optics, epidemiology (multiple foci of infection), and similarly technical scientific writing; use "focuses" for virtually everything else, including business, psychology, and everyday conversation about priorities or areas of attention.

Unlike "cactus," where both plurals compete fairly evenly across registers, "focus" shows an unusually sharp divide: "foci" almost never appears outside technical writing, and "focuses" almost never appears within it, making the choice here more clearly rule-governed by field than by general stylistic preference. The camera and optics industry adds another wrinkle: a lens can have multiple focal points depending on design, and technical writing about optical systems still generally reaches for "foci" when discussing them, while a casual photography blog describing someone's shifting creative "focuses" over a career would never use the Latin form — the same technical/casual split playing out within a single broader field.

Common real mistakes

Writers sometimes assume "foci" is always the more correct or formal choice and use it in general writing where "focuses" would be expected and more natural — reaching for the Latin plural in the wrong register can actually read as more awkward than simply using "focuses."

The wider pattern

"Focus" belongs to the same broad family of Latin -us nouns as "cactus," but its specific declension pattern and its unusually sharp technical/general register split set it apart from "corpus" and "genus," whose Latin plurals follow entirely different declension patterns ("corpora" and "genera," not "-i"). Business and management writing has embraced "focuses" so thoroughly that phrases like "our three strategic focuses this quarter" now sound entirely natural in corporate reports, a register where "foci" would strike most readers as needlessly stiff or overly academic for what is essentially casual planning language.

Examples

• The ellipse has two foci, each equidistant from every point on the curve.
• Epidemiologists identified three separate foci of the outbreak within the county.
• Her career has had several distinct focuses over the past two decades.
• The meeting covered too many focuses to be genuinely productive.
• Astronomers calculated the foci of the comet's elongated orbit.
• The company narrowed its focuses down to two core product lines.
• Radiologists located multiple foci of inflammation in the scan.
• Her thesis examines the shifting focuses of environmental policy since 1970.
• The lens has two foci depending on which side light enters first.
• Team leads were asked to rank their departments' focuses for the coming year.
• The telescope's foci were recalibrated after the lens was replaced.
• Her research shifted through three distinct focuses over ten years.
• Doctors identified multiple foci where the infection had spread.
• The workshop asked each team to narrow their focuses to one goal.
• The startup narrowed its focuses to just two product lines this year.
• Astronomers recalculated the comet's orbital foci after new observations.
• The therapy session covered several unresolved focuses from childhood.
• The company's marketing focuses shifted dramatically after the leadership change.