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.”

A point of a conic at which rays reflected from a curve or surface converge.

The fact of the convergence of light on the photographic medium.

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

(followed by on or upon) To concentrate one's attention.

“I have to focus on my work.”

To cause (rays of light, etc) to converge at a single point.

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.”