(countable) A point at which reflected or refracted rays of light converge.
(countable) A point of a conic at which rays reflected from a curve or surface converge.
(uncountable) The fact of the convergence of light on the photographic medium.
(uncountable) The quality of the convergence of light on the photographic medium.
(uncountable) Concentration of attention.
(countable) Something to which activity, attention or interest is primarily directed.
(countable) The exact point of where an earthquake occurs, in three dimensions (underneath the epicentre).
(countable, uncountable) The status of being the currently active element in a user interface, often indicated by a visual highlight.
(countable, uncountable) The most important word or phrase in a sentence or passage, or the one that imparts information.
(countable, uncountable) An object used in casting a magic spell.
(countable, uncountable) The centre of an older fish's scale, which is the point where a younger fish's scale starts to grow from.

(transitive) To cause (rays of light, etc) to converge at a single point.
(transitive) To adjust (a lens, an optical instrument) in order to position an image with respect to the focal plane.
(intransitive, usually) To adjust itself or be adjusted such that light from a scene converges appropriately to create a clear image.
(transitive) To direct attention, effort, or energy to a particular audience or task.
(intransitive) To concentrate one’s attention.
(intransitive) To concentrate one's attention on something; to have as one's central point of interest, concern, etc.
(transitive) To transfer the input focus to (a visual element), so that it receives subsequent input.
To aggregate figures of accounts.

foci

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