Noun

1. 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. a point of a conic at which rays reflected from a curve or surface converge.
3. The fact of the convergence of light on the photographic medium.

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

4. 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. concentration of attention.

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

6. 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.

7. The indicator of the currently active element in a user interface.

   Text entered at the keyboard or pasted from a clipboard is sent to the component which currently has the focus.

Verb

1. To cause (rays of light, etc) to converge at a single point.
2. To adjust (a lens, an optical instrument) in order to position an image with respect to the focal plane.

   You'll need to focus the microscope carefully in order to capture the full detail of this surface.

3. To concentrate one's attention.

   Focus on passing the test.

4. To make (a liquid) less diluted.
5. To concentrate one’s attention.

   If you're going to beat your competitors, you need to focus.

Usage notes