The term adaptation when used within the context of the biology of the eye refers to its ability to adjust to various levels of darkness and light.

The human eye can function from very dark to very bright levels of light - its sensing capabilities reach across nine decades. This means that the brightest and the darkest light signal that the eye can sense are a factor of roughly one billion apart. However, in any given moment of time, the eye can only sense a contrast ratio of one thousand. What enables the wider reach is that the eye adapts its definition of what is black. The light level that is interpreted as "black" can be shifted across six decades - a factor of one million. If you move from bright sunlight to a completely dark room, it will take your eye about half an hour to adapt to maximum sensibility - one million times more sensible than at full daylight. In this process, the eye's perception of color changes as well.