The “{Hawthorne effect}” is a confounding effect in observational studies: an observed change in behavior caused by a subject’s awareness of being observed (and not by whatever intervention was supposedly being measured).
It’s named after a factory in which experimenters adjusted the light levels of the workshop and evaluated the changes’ effects on productivity—only to find that making any change at all (in either direction) improved their productivity.
The term’s sometimes used in a broader sense, to describe short-term changes in observed behavior caused by (any) change in environment—i.e. any novelty effect.
The empirical data around this effect is apparently controversial and not well-replicated.