ACCLIMATIZATION

Acclimatization (UK also acclimatisation; US also acclimation) is the process in which an individual organism adjusts to a gradual change in its environment (such as a change in temperature, humidity, photoperiod, or pH), allowing it to maintain performance across a range of environmental conditions

Most of the plants that we use indoors were grown in environments very different than the one indoors- there was more sunlight, humidity, perhaps they grew in a rain forest...how do they adapt to the change?

Plants grown in high light situations have more chloroplasts which causes them to have thicker leaves. When they switch to being grown indoors they must make new leaves with less chloroplasts (thinner) so they can photosynthesize more efficiently. (this may include a lot of leaf-drop initially) When changing environments it is better to decrease the light gradually so as to not shock the plant.
When homeowners bring plants home they should at first place them in higher light areas and then gradually move them into their permanent home. 
This also needs to be taken into consideration when taking plants indoors for the winter or outdoors for the summer. Because plants get used to conditions in a certain place it is better to gradually get them used to conditions in the new place.

http://aggie-horticulture.tamu.edu/archives/parsons/publications/houseplant/houseplant.html

On moving plants indoors or outdoors:
http://www.apartmenttherapy.com/acclimatization-tips-bringing-147047

Research on Acclimatization:
http://mrec.ifas.ufl.edu/Foliage/Resrpts/rh_91_5.htm