Lens Hood
A shade fitted to the front of a lens to block light from outside the angle of view before it can strike the front element.
In depth
A hood is the cheapest contrast improvement available, and it works even on overcast days when there is no obvious sun to shield. Petal-shaped hoods are cut to match a rectangular frame, so they must be mounted in the correct orientation or they will vignette. They also provide genuine physical protection for the front element.
Examples in practice
Shooting a street scene with a hood fitted keeps blacks solid even with shop lighting spilling in from the side.