Etimology
Borrowed from Latin focus (“hearth, fireplace”); see there for more. Doublet of fuel. Kepler introduced the term into mathematics and the sciences in describing elliptical orbits of planets (quote from Nicholas Mee) : "One of the interesting properties of an ellipse is that if there were a light bulb at one focus, then all the light that it emits would reflect off the ellipse and converge at the other focus. This is why Kepler originally used the name focus for these points."
noun
focus (countable and uncountable, plural foci or focuses or focusses)
(countable, optics) A point at which reflected or refracted rays of light converge.
(countable, geometry) A point of a conic at which rays reflected from a curve or surface converge.
(uncountable, photography, cinematography) The fact of the convergence of light on the photographic medium.
(uncountable, photography, cinematography) 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, seismology) The exact point of where an earthquake occurs, in three dimensions (underneath the epicentre).
(graphical user interface) The status of being the currently active element in a user interface, often indicated by a visual highlight.
(linguistics) The most important word or phrase in a sentence or passage, or the one that imparts information.
An object used in casting a magic spell.
Examples
Synonym: focal point
The heat of sunlight at the focus of a magnifying glass can easily set dry leaves on fire.
Unfortunately, the license plate is out of focus in this image.
Not long ago, it was difficult to produce photographs of tiny creatures with every part in focus. […] A photo processing technique called focus stacking has changed that. Developed as a tool to electronically combine the sharpest bits of multiple digital images, focus stacking is a boon to biologists seeking full focus on a micron scale.
During this scene, the boy’s face shifts subtly from soft focus into sharp focus.
I believe I can bring the high degree of focus required for this important job.
Synonym: focal point
My studies are my main focus right now.
The earthquake's focus was at exactly 37 degrees north, 18 degrees south, seventy five meters below the ground.
Text entered at the keyboard or pasted from a clipboard is sent to the component which currently has the focus.
Candles, in fact, are an essential ingredient in many spells. They can be used as either the focus of the spell or as a component that sets the spell's overall mood and tone.
I ran through what I knew about spells from Mom and other witchcraft sources, but nothing matched what I was used to seeing in her magic work. Usually she used herbs and other plants as a focus for the spell.
verb
focus (third-person singular simple present focuses or focusses, present participle focusing or focussing, simple past and past participle focused or focussed)
(transitive) To cause (rays of light, etc) to converge at a single point.
(transitive, optics) To adjust (a lens, an optical instrument) in order to position an image with respect to the focal plane.
(intransitive, optics, of a lens, optical instrument, etc.) 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, followed by on or upon) To concentrate one's attention on something; to have as one's central point of interest, concern, etc.
(computing, graphical user interface, transitive) To transfer the input focus to (a visual element), so that it receives subsequent input.
(accounting, formerly) To aggregate figures of accounts.
Examples
The difficulties of focussing colour-light signals on curved tracks to ensure maximum sighting distance were underlined in the recent official report [...] on a low-speed collision at Newcastle Central on July 25, 1960, between an unfitted freight and a diesel-hauled passenger train.
You'll need to focus the microscope carefully in order to capture the full detail of this surface.
I can't get the lens to focus.
The camera focuses automatically on the subject's eyes.
The president focused her remarks on the newcomers.
If you're going to beat your competitors, you need to focus.
I have to focus on my work.
The text box won't receive the user's keystrokes unless you explicitly focus it.
Whole pages of it are filled with masses of figures, generally single numbers added up in batches, and then the totals added in batches again, as though he were "focusing" some account, as the auditors put it.
Related words
related terms
focal