Etiolation is the reaction of a plant to insufficient light: its stems stretch and become thinner, leaning towards the light source. Its leaves tend to be smaller or thinner and are spaced more widely apart along the stem than normal. Also, new leaves and stems will be a paler green or even, in almost complete absence of light, pale yellow, pale pink, or white.
In general, etioliation is unwanted and is corrected by moving the plant to a location where it will receive more light, but sometimes gardeners use it to create a desirable effect. They place the endives, for example, in total darkness, causing them to turn nearly white, as this reduces the natural bitterness of their leaves.