noun pl. focuses fo′·cuses or foci fo′ci′ (-sī′)

1. the point where rays of light, heat, etc. or waves of sound come together, or from which they spread or seem to spread; specif., the point where rays of light reflected by a mirror or refracted by a lens meet (called real focus) or the point where they would meet if prolonged backward through the lens or mirror (called virtual focus)
2. focal length
3. an adjustment of the focal length to make a clear image to bring a camera into focus
4. any center of activity, attention, etc.
5. a part of the body where a disease process, as an infection, tumor, etc., is localized or most active
6. the starting point of an earthquake
7. Math.
   1. either of the two fixed points used in determining an ellipse
   2. any analogous point for a parabola or hyperbola

Etymology: ModL, adopted in math. senses by Johannes Kepler (1604) < L, fireplace, hearth < ? IE base *bhok-, to flame, burn > ? Arm bo, flame

transitive verb focused -·cused or focussed -·cussed, focusing -·cus·ing or focussing -·cus·sing

1. to bring into focus
2. to adjust the focal length of (the eye, a lens, etc.) in order to produce a clear image
3. to fix or settle on one thing; concentrate to focus one's attention on a question

intransitive verb

1. to meet at a focus
2. to adjust one's eye or a lens so as to make a clear image
3. to direct one's thoughts or efforts; concentrate