The potential for bias in an experiment or observational study caused by the possibility that subjects may be influenced by the fact that they are being observed.
(Phil Oreopoulos, Toronto PPG 1004H)
The name comes from a classic study in which researchers were studying the effect of lighting on worker productivity. As the intensity of the factor lights increased, so did the work productivity. One researcher suggested that they reverse the treatment and lower the lights. The productivity of the workers continued to increase. It appears that being observed by the researchers was increasing productivity, not the intensity of the lights.