Foci

Formation of nuclear "foci"

Noun - focus, Plural - countable and uncountable; foci or focuses

• 1. (countable) (optics) a point at which reflected or refracted rays of light converge.

The heat of sunlight at the focus of a magnifying glass can easily set dry leaves on fire.

• 2. (countable) (geometry) a point of a conic at which rays reflected from a curve or surface converge.
• 3. (uncountable) (photography, cinematography) The fact of the convergence of light on the photographic medium.

Unfortunately, the license plate is out of focus in this image.

• 4. (uncountable) (photography, cinematography) The quality of the convergence of light on the photographic medium.

During this scene, the boy’s face shifts subtly from soft focus into sharp focus.

• 5. (uncountable) concentration of attention.

I believe I can bring the high degree of focus required for this important job.

• 6. (countable) the exact point of where an earthquake occurs, in three dimensions.

The earthquake's focus was at exactly 37 degrees north, 18 degrees south, seventy five meters below the ground.[1]