Wikipedia
In photography, a lens hood or lens shade is a device used on the front end of a lens to block the Sun or other light source(s) to prevent glare and lens flare. Lens hoods may also be used to protect the lens from scratches and the elements without having to put on a lens cover.
The geometry of a lens hood is dependent on three things: the focal length of the lens, the size of the front lens element and the dimensions of the image sensor or film in the camera.
Usage examples of "lens hood".
Swinging his telephoto lens to the left--making sure the lens hood was extended to keep out the low-slanting sun this time--Tommy checked on the whereabouts of the other campers.
I removed the lens hood for him and he took the camera and stood up, putting his eye to the rubber eyecup and then slowly covering the park in a virtuoso 36o-degree pan.
This one, I'm afraid, is virtually useless: in spite of its lens hood the rain is driving straight into the lens.
He clipped on a lens hood and took one picture to see that all was working.